Which European leader stopped Germany's attempted annexation of Austria in 1934?

Anschluss, the idea of uniting Austria and Germany into a Greater Germany, was a political ideology that had emerged in the latter 19th century with the unification of Germany. With the dissolution of the Austrian-Hungarian Empire after WW I, the push for unification accelerated after Hitler's rise to power in Germany in 1933. A Greater Germany was a major principle of Nazi ideology. Austria was a natural candidate for annexation since it had a strong German cultural history, Hitler, himself, was Austrian by birth. There was also a strong movement for Austria to remain independent

During an attempted coup in 1934, Austrian German SS men disguised as Austrian soldiers assassinated the Austrian Prime Minister Englebert Dollfuss. Despite internal Austrian support for the Nazis, there was enough domestic opposition against Anschluss. The push failed especially after Mussolini mobilized troops along the Austrian-Italy border and warned Hitler not to invade Austria.

By 1938, Hitler and Mussolini had come to an understanding and Austria was annexed by Germany. The first foreign conquest that led to WW II.