Why was Adolf Hitler's foreign policy successful up to the 1938 Anchluss with Austria?
The reason German foreign policy could be considered successful from the date of Adolf Hitler's appointment as chancellor of Germany to the March 1938 union with Austria was because there was relatively little to prevent it from carrying out its chosen policies. The 1930s were an economically turbulent time for much of Europe, as well as for the United States. The Great Depression was not limited to the United States; on the contrary, much of the world felt its effects, and many governments were preoccupied with adopting measures intended to lift themselves out of the depths to which they had descended. In addition, the rise of Nazi Germany was not occurring in a vacuum. Benito Mussolini was already entrenched as the leader of Fascist Italy. In the Spanish Civil War, which began in 1936, the Nationalists, a fascist movement led by Francisco Franco, defeated the Republicans, a group supported by socialist and communist movements from across Europe (and the United States). Imperial Japan had invaded and colonized Manchuria. Germany was active in undermining the egregious terms of the Treaty of Versailles, under which it was required to cede territory, pay reparations, and limit its military as a result of its loss in the WWI. Most importantly, many European leaders lacked the wisdom and will to confront Hitler even as his transgressions, including the implementation of laws designed to marginalize and eventually destroy the nation's Jewish population, continued to grow in scale.
Germany, then as now, is a very powerful nation, technologically advanced and with a large population, and the governments of France and Great Britain failed to appreciate the direction in which Hitler was dragging the continent. German withdrawal from the stillborn League of Nations in 1933 and its determination to rebuild its armed forces were certainly viewed as potentially problematic, but it was able to justify its 1936 remilitarization of the Rhineland region as a reaction to France's agreement with the Soviet Union the previous year (German views of the Soviet Union, for both racial and ideological reasons, were entirely hostile). That justification aside, there is no question that Hitler would have ordered the remilitarization of that vital region anyway given his determination to make Germany the greatest military power in Europe, and the failure of France and Great Britain to respond to that major transgression of the Treaty of Versailles presaged greater displays of appeasement later on.
German foreign policy was so successful between 1933 and 1938 because Hitler was able to present his country as more powerful than it actually was, at the time, due in no small part to that country's reputation for military prowess, and by European leaders' unwillingness to force the issue with Berlin. As Germany methodically rebuilt its armed forces, there was no real resistance from France and Great Britain (and, it should be noted, the French fascist movement friendly to Nazi Germany was itself gaining in strength). The remilitarization of the Rhineland, in particular, should have been a wake-up call, but not even the post-Anschluss provocations, like Germany's seizure of the Sudenten region of Czechoslovakia, would prove sufficient to compel a major response from the British and French governments. The French and British were so opposed to any policies that could lead to another war that their inaction helped cause the worst, most destructive war in history.
At the end of the day, the reason for Hitler's success in his conduct of foreign policy was the reluctance of the major powers of Europe, France and Great Britain, to forcefully oppose him.
Hitler's foreign policy during this time was successful because none of the Allied powers was willing to fight to stop him. It was also a success because some of the Allies, at least, felt that the Treaty of Versailles was too harsh. This was one reason that they did not try to stop Hitler when he broke that treaty.
Prior to the Anschluss, Germany did little that seemed worth fighting over. It did things like pulling out of the League of Nations and creating an alliance with Italy and Japan that was supposed to be based on anti-communism. It remilitarized the Rhineland, which was, after all, its own territory. None of those things was likely to inspire countries like England and France to go to war when they were so traumatized by what had happened in WWI.
The most provocative thing that Germany did was to participate in the Spanish Civil War. However, they got in on the anti-communist side. This helped make their actions seem legitimate to many in the international community who feared communism.