Vansittart was suspicious of Hitler from the start; anything Hitler said, he claimed, was "for foreign consumption" and thought he would start another European war as soon as he "felt strong enough". Vansittart believed in revising the Versailles Treaty in Germany's favour but not while Hitler was in power. In Vansittart's view, Britain should be firm with Germany, and an alliance between France and Russia against Germany was essential. Vansittart also urgently advocated rearmament.
In the summer of 1936 Vansittart visited Germany and claimed that he found a climate that "the ghost of Barthou would hardly have recognised" and that Britain should negotiate with Germany. He thought that satisfying Hitler's "land hunger" at Russia's expense would be immoral and regarded the Franco-Russian alliance as non-negotiable. It was because he believed Germany had gained equality in Europe that Vansittart favoured facilitating German expansion in Africa. He thought that Hitler was exploiting fears of a "Bolshevist menace" as a cover for "expansion in Central and South-Eastern Europe".
Like Maurice Hankey, Vansittart thought in power politics terms. He thought Hitler could not decide whether to follow Goebbels and Tirpitz in viewing Britain as "the ultimate enemy" or on the other hand adopting the Ribbentrop policy of appeasing Britain in order to engage in military expansion in the East. Vansittart thought that in either case time should be "bought for rearmament" by an economic agreement with Germany and by appeasing "genuine grievance[s]" about colonies.
Vansittart wanted to detach Mussolini from Hitler. He thought that the British Empire was an "Incubus" and that the Continent was the central British national interest, but he doubted whether agreement could be had there. This doubt rested on his fear that German attention, if turned eastwards, would result in a military empire between the Baltic, the Adriatic and the Black Sea.
At the Foreign Office in the 1930s, Vansittart was a major figure in the loose group of officials and politicians opposed to appeasement. He was also involved in intelligence work. He thinking was along the same lines as Churchill.
During the war, Vansittart became a prominent advocate of an extremely hard line with Germany. His earlier worries about Germany were reformulated into an argument that Germany was intrinsically militaristic and aggressive. In Black Record: Germans Past and Present (1941), Vansittart portrayed German history from the time of ancient Rome as a continuous record of aggression.
Nazism was just the latest manifestation. Therefore, after Germany was defeated, it must be stripped of all military capacity, including its heavy industries. The German people enthusiastically supported Hitler's wars of aggression, just as they supported the Franco-Prussian War in 1870 and World War I in 1914. So they must be thoroughly re-educated under strict Allied supervision for at least a generation. De-Nazification was not enough. The German military elite was the real cause of war, especially the "Prussianist" officer corps and the General Staff: both must be destroyed. In 1943 he wrote:
In the opinion of the author, it is an illusion to differentiate between the German right, center, or left, or the German Catholics or Protestants, or the German workers or capitalists. They are all alike, and the only hope for a peaceful Europe is a crushing and violent military defeat followed by a couple of generations of re-education controlled by the United Nations.
This doctrine, which was known as "Vansittartism", was quite influential in Britain and the U.S. The Morgenthau Plan drew on it, and Vansittartist attitudes helped obstruct contacts between the Western Allies and the German Resistance. On the other hand, Vansittartism was denounced by many left-wing figures as blaming the German working class for crimes of which they were the first victims. The socialist publisher Victor Gollancz was perhaps the strongest critic.