Instead he enlisted in the French Army as a private soldier in 1831 with a view to service in Algeria, where in 1833 he received a commission as sub-lieutenant in the Foreign Legion, it having been formed by Louis Philippe (King of France 1830-1848) in 1831. He rose rapidly through the ranks (Lieutenant 1835, Captain 1837), through successful actions during the Foreign Legion campaign in Algeria and against the Carlists in Spain (1835-1839), where in 1835 he was cited for bravery and gallantry in action on several occasions and rewarded with the cross (Chevalier) of the Légion d'honneur after only four years in the Army. After serving a second campaign with the Foreign Legion in Spain in 1837-38 (wounded: bullet in the right leg, Battle of Barbastro, 1837), Bazaine returned to Algeria in 1839 and took part in the expeditions to Milianah, Tlemcen, Morocco and Sahara. He was mentioned as instrumental in the surrender of Abd-el-Kader. In 1844 he was promoted to Major (wounded: bullet in the right wrist during action at Macta, 1845) and then to Lieutenant Colonel in 1848 after 9 years service in Algeria and Morocco, including several years heading France's Bureau Arabe (military intelligence) as Governor of Tlemcen. In 1850, he was promoted to full Colonel and given command of the 1st Regiment of the Foreign Legion, based in North Africa. He married his first wife Maria Juana de la Soledad, on 12 June 1852 at Versailles.
In 1859, he commanded a Division in the Franco-Sardinian campaign against Austrian forces in Lombardy. He was wounded by a shell splinter in the head on 8 June, during the action at the Battle of Melegnano. He recovered to play a conspicuous part in the Battle of Solferino, which he captured on 24 June 1859, despite being wounded again (bullet to the upper thigh) and having his horse shot from under him. For his services in the campaign he became a Knight of the Légion d'honneur, of which he was already (1855) a Commander.
He commanded with great distinction the First Division under General (afterwards Marshal) Forey in the Mexican expedition in 1862, where he pursued the war with great vigour and success, driving President Benito Juárez to the frontier. His decisive action was instrumental in the taking of the city of Puebla in 1863. In the same year, he was cited again for his bravery in the Battle of San Lorenzo, for which he was made Knight Grand Cross of the Legion of Honour on 2 July 1863. On 5 September 1863 he was raised to Marshal of France by Presidential decree and elected to the senate. At the same time he replaced Forey in supreme command. He personally commanded the siege of Oaxaca in February 1865, following which the Emperor Maximilian decorated him with the Medaille Militaire on 28 April 1865. Here as in 1870, two of Bazaine's nephews, Adolphe and Albert Bazaine-Hayter served with their uncle as his aides-de-camp. The Marshal's African experience as a soldier and as an administrator stood him in good stead in dealing with the guerrilleros of the Juárez party, but he was less successful in his relations with Maximilian, with whose court the French headquarters was in constant strife. His enemies whispered that he aimed to depose Maximilian and get the throne of Mexico for himself. or that he aspired to play the part of a Bernadotte. His marriage to a rich Mexican lady (Pepita de la Peña y Azcarate), whose family were supporters of Juárez, still further complicated his relations with the unfortunate emperor, and when at the close of the American Civil War the United States sent a powerful war-trained army to the Mexican frontier, Napoleon III commanded Bazaine to withdraw French forces and return to France. Bazaine skillfully conducted the retreat and embarkation at Veracruz (1867). On his return to Paris he was feted by the public. Bazaine took his seat in the Senate as a Marshal of France and was appointed Commander-in-Chief of the Imperial Guard in Paris.
It is clear even at this early stage that Bazaine was acutely aware of his Army's shortcomings against the well known speed and menacing efficiency of the Prussian military machine, evidenced by his remark to a friend whilst boarding the train from Paris to Metz: "Nous marchons à un désastre". He had absorbed certain lessons that were to become a vital part of French military thought. From the story of Waterloo he had learned that a line of resolute men on the defensive could again and again break an enemy attack. From Mexico he had watched Lee's dashing Confederates lose a war despite their commander's brilliance in attack. He had also learned that dramatic sorties were invaluable in North Africa but were risky against European armies. Finally, Bazaine saw with misgivings the Prussian invention all-steel Krupp breech-loading gun, which was to shape the future of artillery on the battlefield. He concluded at this time that for France defensive war is better than offensive war. "It is better," he said, "to conduct operations systematically (i.e., defensively], as in the Seventeenth Century..
Bazaine took no part in the earlier battles, but after the defeats of Marshal MacMahon’s French Forces at Worth and Marshal Canrobert’s at Forbach, Napoleon III (who was increasingly unwell), was swift to hand over to Bazaine as Commander-in-Chief of the French Army on 13 August 1870. Napoleon’s choice was at the time, considered to be a wise one. It was widely believed by French politicians and soldiers alike, that if any one was capable of saving France from the Prussian onslaught, “notre glorieux Bazaine” was (Gambetta). He was also the only remaining Marshal of France not to have suffered defeat at the hands of Prussian forces in the early weeks of the war. However, being the youngest of the French Marshals, Napoleon’s choice was met with suspicion and jealousy by the older, socially superior Marshals. It was reluctantly therefore, that he took up the chief command, and his tenure of it is the central act in the tragedy of 1870. He found the army in retreat, ill-equipped and numerically at a great disadvantage, and the generals and staffs discouraged and distrustful of one another. There was practically no chance of success. The question was one of extricating the army and the government from a disastrous adventure, and Bazaine's solution of it was to bring back his army to Metz. The day after assuming command of the Army, on 14th August at Borny, he was badly wounded by a shell on the left shoulder (a fact which was to be excluded from his service roll presented at his Court Martial in 1873, there was to be no room for sympathy.)
It seems to be clearly established that the charges of treason had as yet, no foundation in fact. Nor, indeed, can his unwillingness to leave the Moselle region, while there was yet time to slip past the advancing enemy, be considered even as proof of special incompetence. The resolution to stay in the neighbourhood of Metz was based on the knowledge that if the slow-moving French army ventured far out it would infallibly be headed off and brought to battle in the open by superior numbers. In "strong positions" close to his stronghold, however, Bazaine hoped that he could inflict damaging repulses and heavy slaughter on the ardent Germans, and in the main the result justified the expectation. The scheme was creditable, and even heroic, but the execution throughout all ranks, from the Marshal to the battalion commanders, fell far short of the idea. The minutely cautious methods of movement, which Algerian experience had evolved suitable enough for small African desert columns, which were liable to surprise rushes and ambushes, reduced the mobility of a large army, which had favourable marching conditions, to 5 miles a day as against the enemy's rate of 15. When, before he had finally decided to stay in Metz, Bazaine attempted halfheartedly to begin a retreat on Verdun, the staff work and organization of the movement over the Moselle was so ineffective that when the German staff calculated that Bazaine was nearing Verdun, the French had in reality barely got their artillery and baggage trains through the town of Metz. Even on the battlefield the Marshal forbade the general staff to appear, and conducted the fighting by means of his personal orderly officers.
Up to this point Bazaine had served his country perhaps as well as circumstances allowed, and certainly with enough skill and a sufficient measure of success to justify his appointment. His experience, wide as it was, had not fitted him for the command of a large army in a delicate position. Since the start of the war, Bazaine appeared to lack the appetite for the fight which had been his trademark in his military career to date, and although imperceptible on the field of battle, because his reputation for impassive bearing under fire was beyond question, was only too obvious in the staff offices, where the work of manoeuvring the army and framing plans and orders was chiefly done. In spite of this, it cannot be asserted that any of Bazaine's subordinates would have done better.
The scheme, however, collapsed and Bazaine surrendered the Army of the Rhine who became prisoners of war to the number of 180,000. This surrender is often explained by Bazaine's lack of motivation to defend a government that corresponded less and less to his political ideals and the best interests of France, as he saw it. At the moment of the surrender a week's further resistance would have enabled the levies of the National Defence government to crush the weak forces of the Germans on the Loire and to relieve Paris. But the army of Prince Frederick Charles, set free from the siege of Metz by Bazaine's surrender, hurried up in time to check and to defeat the great effort at Orléans.
When Bazaine returned from captivity, aware that in his absence he had been put forward as a scapegoat by the new government of the Third Republic, for France's defeat at the hands of the Prussians, was keen to be given an opportunity to clear his name and put his version of events to the public. In 1872, Bazaine published his account of the events of 1870 in L'Armée du Rhin and formally requested and was granted a trial before a military court. For months he was retained a prisoner at Trianon Palace, Versailles with his wife and two youngest children, while preparations were made for the great court-martial spectacle, which started the following year on 6 October 1863 under the presidency of the Duc D'Aumale in the Gallery of Trianon Palace.
For some time the Duke and his colleagues had been looking for a way out of their difficulty, by which they could save themselves, satisfy public clamor and yet avoid responsibility before history. Bazaine stated in his defence "I have graven on my chest two words - Honour and Country. They have guided me for the whole of my military career. I have never failed that noble motto, no more at Metz than anywhere else during the forty-two years that I have loyally served France. I swear it here, before Christ". Despite a vigorous defence of Bazaine's actions by Lachaud, and the presentation of a number of strong witness statements from his staff including Colonel Willette, the court found Bazaine guilty of negotiating with and capitulating to the enemy before doing all that was prescribed by duty and honour. It was clear even to the most partial observer, that the verdict bore very little relation to the evidence. For example, the Marshal surrendered only after receiving letters recommending him to do so from his Generals, but the presentation of these at the trial was ignored. "I have read every word of the evidence [against Bazaine] and believe it to be the most malicious casuistry" (New York Times Correspondent). A letter which Prince Frederick Charles wrote in Bazaine's favor only added to the wrath of the people, who cried aloud for his execution. The court sentenced Bazaine to 'degradation and death', and to pay the costs of the enormous trial (300,000 Francs), which was to leave the Marshal's young family penniless. Bazaine's reaction on being read the sentence of the court was "It is my life you want, take it at once, let me be shot immediately, but preserve my family". Since the Revolution, only two French Marshals have been condemned to death - Ney, by a Bourbon, and Bazaine, by an Orléans. But, as though the judges themselves felt a twinge of conscience at the sentence, they immediately and unanimously signed a petition for 'Executive Clemency' to the President of the Third Republic, Marshal MacMahon, although Bazaine refused to sign this petition himself.
MacMahon, who was a fellow Foreign Legion Officer and had served in many campaigns alongside Bazaine, was visibly disgusted when he received the news of the Court's decision and was incensed by their attempt to pass responsibility to him. The government wanted to banish Bazaine for life; MacMahon first proposed life imprisonment, though he softened and commuted the punishment of death to twenty years' imprisonment and remitted the disgrace of the formalities of a military degradation. Bazaine wrote to thank his fellow legionnaire, though he added, tongue in cheek, that he might have let his feelings run away with him. It was an academic concession for a man nearing sixty-three. Bazaine was incarcerated on the Île Sainte-Marguerite and treated rather as an exile than as a convict. During the night of 10 August 1874, using parcel rope supplied by Angelo Hayter, (son of the Court Painter Sir George Hayter) and baggage straps which he knotted in to a rope, the 63-year-old Marshal attached one end to his body and tied the other end to a gargoyle and climbed down the 300 foot cliffs to a boat which his wife had brough out from Cannes. They sailed to Genoa Italy, and from there Bazaine came to London with his young family where he stayed for a time with his Hayter relations.
In the same year as Bazaine’s death, Count d’Herrison published an account in defence of the Marshal’s decisions during the Franco-Prussian war, which cast significant, verifiable doubt upon the characters and motivations of witnesses whose testimonies were key to the finding of the court that Bazaine was guilty of treason. Between 1904 and 1912, the French Court of Appeal lawyer Élie Peyron published several works in defence of Bazaine including , and .
"MacMahon, the aristocrat survived Bazaine by five years; Paris gave MacMahon a funeral that choked the wide boulevards for hours. Canrobert, last of the Foreign Legion Marshals of the Second Empire, was buried like a prince in 1895. The Foreign Legion, which has never felt obliged to accept the French view on anything, still honours Bazaine. In its museum there exists almost no trace of MacMahon, nor of Canrobert or of Saint-Arnaud. Bazaine however has his own corner, adorned with his battered kepi, the bits and pieces of the harness he used at Rezonville and Gravelotte, and the cross Conrad pinned on him after Macta. The Legion knows that courage is not a mask that a soldier can wear or discard at will. To this day, the Legion annually pays tribute to Bazaine's courage.