Why was Hitler's foreign policy successful up to 1938? Hitler's actions seemed reasonable; give examples of actions that he took which seemed to have good reason (e.g., rearmament).

In addition to rearmament, the Third Reich made other foreign policy moves that could be considered reasonable—or, in any case, rationalized.
In October 1933, Germany withdrew from the League of Nations under the contention that the League was not allowing it to achieve equal military strength with other member nations. This move could be seen as reasonable by casting the League's policy as unfair.
In 1935, the Saar Valley was folded back into Germany. This was reasonable, as the territory was only supposed to be separate as a territory for fifteen years after World War I. Due to an overwhelming number of the residents approving the move, the territory was reunited with Germany. This aligned completely with Hitler's foreign policy objective of reuniting all German people under one flag.
In 1936, the German army invaded the demilitarized Rhineland, justifying seizing it by saying it feared a recent treaty between France and the USSR.
In 1938, Hitler invaded Austria to insure the Anschluss, or uniting of the two German-speaking countries. This violated the Treaty of Versailles, but Hitler said the two countries shared a common heritage and destiny.
Hitler's foreign policy was successful in the 1930s because the powers that could have easily opposed his territorial ambitions remained silent. In part, this appeasement was because France and Great Britain feared the USSR and wanted a strong Germany as a buffer between them and the Communists.