What major political agreement was signed in August 1939 that shocked many due to the alliance between former rivals?

The major political agreement signed in August 1939 that shocked many due to the unexpected alliance between former rivals is the Nazi-Soviet Pact, also known as the Molotov-Ribbentrop Pact. This non-aggression treaty was between Nazi Germany and the Soviet Union, and its signing was surprising because the two powers had stark ideological differences; one was a fascist regime while the other was a communist state. The pact allowed both nations to divide Eastern Europe between them, specifically Poland, which indicated a temporary cessation of hostilities and a mutual interest in territorial expansion.

The agreement not only facilitated Hitler’s invasion of Poland in September 1939 but also allowed Stalin to reclaim territories lost during World War I and expand Soviet influence in the Baltic States. The shock stemmed from the fact that both countries had been opposed to each other in previous conflicts—particularly during the Russian Civil War and in their contrasting philosophies regarding governance and society.

The other options reflect different historical agreements that did not create the same level of astonishment or alliance between such ideologically opposed regimes at that time.