Immediately after the World War I, the peace treaties recognized the reestablishment of a Polish state for the first time in over 100 years. Romania emerged from the war as a victorious nation, enlarging its territory (as Greater Romania). Both states had serious reasons to stand by these treaties.
Having established contacts with Poland in January-February 1919 (after Stanisław Głąbiński's visit to Bucharest), Romania oriented itself towards a cordon sanitaire alliance aimed at Bolshevist Russia and the newly-created Comintern; the proclamation of the Hungarian Soviet Republic, the German insurrection, and the Red Army's capture of Odessa alarmed politicians in both countries. The diplomat Czesław Pruszyński reported to the Polish government:
"A dam that can put a stop to Bolshevik pressure on the West is constituted of Poland to the north, and Romania to the south. [...] There is a natural necessity, but also a historical necessity, that, based on the mutual interests of Romania and Poland, a military alliance be sealed in front of the common threat facing them.
Romania was not engaged in the Polish-Soviet War, but accepted and supported Polish military transit through its territory. According to another of Pruszyński's reports, Romania facilitated the transit of Polish nationals from Russia to their native areas, as well as furnishing armament and grain at preferential prices. In this context, the Romanian Army intervened in the Polish-Ukrainian War against the West Ukrainian People's Republic (created in Galicia in the summer of that year), helping the Poles in Pokuttya (see Romanian occupation of Pokuttya).
Count Aleksander Skrzyński, acting with the acknowledgement of Polish leaders Ignacy Jan Paderewski and Józef Piłsudski, extended an offer to the Romanian government of Ion I. C. Brătianu to participate in the future administration of Ukraine in its entirety (August 1919); the message was again stated after Skrzyński became ambassador in Romania the following month. Alexandru G. Florescu, the ambassador to Warsaw, reported back that the plan for a common military administration was:
"[...] an inaccuracy and a fantasy which I suppose one should not take into account for anything other than making stock of them.
Agreeing with Florescu's assessment, the Brătianu cabinet expressed a will to establish contacts with the Ukrainian People's Republic. In 1920, a similar plan was proposed by Piłsudski himself to the Alexandru Averescu government; the offer was more specific, indicating that Romania was to extend its administration to the east (the Black Sea shore, Odessa, and Transnistria). Averescu refused to accept the proposal, as it meant his country's involvement in the Russian Civil War.
The first treaty was the Convention on Defensive Alliance, signed on March 3, 1921 in Bucharest. The treaty, concluded for a period of five years, committed both parties to rendering armed assistance to one another "in case one of the sides is attacked at its present Eastern frontiers", and was aimed at containing Russia —from, 1922, the Soviet Union—, which had just lost the Polish-Soviet War. Among the diplomats engaged in negotiations were Polish general Tadeusz Rozwadowski and Romanian general Ion Antonescu.
In the early 1920s, Romania, along with Czechoslovakia and Yugoslavia, initiated the Little Entente, an alliance whose primary goal was to counter Hungary's revanchism (which involved Romania's Transylvania). Romania's Foreign Minister, Take Ionescu, attempted to bring Poland and Greece into the alliance, but could not because of the border disagreements between Poland and Czechoslovakia. The Romanian representative in Warsaw reported:
"Poland's policy towards the Little Entente [...] becomes clearer. Poland will not wish to join it. [...] This attitude may be related to Mr. Beneš's, who seems to have declared that Poland's joining the treaty is not currently possible.
However, in 1925, the Locarno Treaties were signed. Through these treaties, Germany was committing to preserve the status quo in the Rhine region. Germany also signed arbitration conventions with Poland and Czechoslovakia, but the Polish government felt betrayed by France.
Under these circumstances, the Polish-Romanian treaty's renewal was being discussed in the early months of 1926. The Romanian foreign minister, Ion G. Duca wrote in a telegram to the Romanian ambassador to Paris:
"Our treaty with Poland expires on the 3rd of March. The Poles will not renew it in the present form, as they have to take into account the atmosphere created by the League of Nations and the Locarno Treaties. They also do not want to keep an exclusively anti-Russian treaty [...] Poland wishes to obtain our help in case it were attacked by Germany.
At this time, in both countries, political changes were taking place. The Romanian king Ferdinand I died in 1927, leaving the throne to his grandson, Mihai. Mihai was, however, underage, and a regency took over the administration of the monarchic institution until the 1930 takeover of Carol II. In Poland, the Sanacja movement took the power after the May coup, marking the beginning of the Piłsudski's dictatorship. On January 15, 1931, Poland and Romania signed a Guarantee Treaty. Around October, rumors spread in Poland that Piłsudski was considering Prince Nicholas, Romania's former regent, for the vacant Polish throne; they were encouraged by conservative politicians such as Janusz Radziwiłł — during the Polish leader's visit to Romania in that month, Radziwiłł spoke in the Senate about the possibility of a monarchic revival (interpreted by Zdzisław Lubomirski to be the result of "an indication coming from [Piłsudski]").
By 1932, Adolf Hitler had come close to taking power in Germany. Therefore, Poland tried to secure its Eastern border by negotiating a treaty with the Soviets and signing a Soviet-Polish Non-Aggression Pact. Romania could not do the same, however, given that the Soviets had not recognized the Soviet-Romanian border on the river Dniester (namely, Romania's rule over Bessarabia). In the same year, Romanian Prime Minister Nicolae Iorga was informed by the ambassador in Warsaw, Grigore Bilciurescu, that conservative groups were considering the possibility of a personal union, with Carol as king of both countries.
During the premiership of Gheorghe Tătărescu (1934-1937), Romania's attempt to balance its alliances with Poland and Czechoslovakia was put to the test by a political scandal, after Jan Šeba, the Czechoslovak ambassador to Bucharest, published a volume overtly sympathetic to a rapprochement between the Soviet Union and the Little Entente, one prefaced by the Czechoslovak Foreign Minister Kamil Krofta; in early 1937, Krofta denied knowledge of the book's content and, after Tătărescu visited Milan Hodža, his counterpart in Prague, Šeba was recalled. Later in the same year, Tătărescu met with the Polish Foreign Minister Józef Beck in Bucharest — the latter, who had previously opposed the pro-status quo policies of Nicolae Titulescu, unsuccessfully proposed that Romania withdraw its support for Czechoslovakia and attempt to reach a compromise with Hungary.
In 1938, in the wake of the Czechoslovak crisis, Beck urged the Romanian government of Miron Cristea, formed by the National Renaissance Front, to participate at the partition of Czechoslovakia (the Munich Agreement), by supporting Hungary's annexation of Carpathian Ruthenia, in the hope that Hungary's Miklós Horthy would no longer sustain claims over Transylvania. The offer was refused by the Romanian king Carol II.
As the situation was becoming increasingly volatile in the eve of World War II, the two countries improved their relations again. In 1938, Richard Franasovici, the Romanian ambassador in Warsaw reported that:
"[There is] an obvious improvement of Poland's sentiments towards Romania [...] The main idea here is maintaining, above everything, the alliance with Romania, of course, due to the growing pressure from Germany, as well as due to the desire to not be completely isolated in the Ukrainian problem [...] Also, [the Poles] consider that the German influence in Budapest and Prague is too powerful [...].
Both countries soon offered each other assistance. After the partition of Czechoslovakia, the Romanian government feared that their country could be next. Ambassador Franasovici reported in March 1939 that:
"[...] as with their appeasing intervention in Budapest, the Polish government pointed out that any action of Hungary against Romania could lead to a new world war, and guaranteed Romania's peaceful intentions.
Polish diplomacy also secured British guarantees to Romania in the Polish-British Common Defense Pact of 1939. Diplomats and strategists in Poland viewed the alliance with Romania as an important part of Polish foreign and defense policy — nevertheless, it eventually proved to be mostly irrelevant. In the period immediately preceding the war, Poland and Romania avoided specifically aiming their agreements against Germany, a country with which both were still seeking a compromise — Beck and Grigore Gafencu agreed on this point after April 1939 negotiations in Kraków).
After the Red Army joined the Nazi attack on September 17, and Western assistance was not forthcoming (see Western betrayal), the Polish high command abandoned this plan, and ordered its units to evacuate to France; many units went through Romanian borders, where they were interned, but Romania remained friendly towards Poles, allowing many soldiers to escape from the camps and move to France, and treating Polish interned soldiers and immigrants with relative respect throughout the war, even when it joined the Axis Powers in 1941 (see Romania during World War II). However, as a result of German pressures, Romania could not openly aid the Poles.
On September 21, Romanian pro-British Premier Armand Călinescu was killed in Bucharest by a squad of local fascist activists of the Iron Guard, acting with Nazi support. Immediately afterwards, German authorities issued propaganda blaming the action on Polish and British initiative; notably, the Nazi journalist Hans Fritzsche attributed the assassination to Polish and British resentments over Romania's failure to intervene in the war.
Poland, for example, had good relations with Latvia, Romania, Hungary and France (Franco-Polish Military Alliance), but poor relations with Czechoslovakia (see Border conflicts between Poland and Czechoslovakia) and Lithuania (see Polish-Lithuanian War); similarly, there were tensions between Hungary on one side and Romania and Czechoslovakia on the other. Such conflicts prevented Poland and Hungary from joining the Little Entente, and over the period of the next two decades the political arena of that region remained filled mostly with treaties and alliances resembling the Polish-Romanian Alliance.