After the failure of the Italian invasion of Greece, Nazi Germany demanded that Bulgaria join the Tripartite Pact and permit German forces to pass through Bulgaria to attack Greece in order to help Italy. The Bulgarian prime minister signed the pact on the 1 March 1941; German forces crossed the Danube into Bulgaria the same day. The threat of a possible German invasion, as well as the promise of Greek and Yugoslavian territories, led the tsar and his government to sign the Tripartite Pact on 1 March 1941. Tsar Boris III and prime minister Bogdan Filov were also both known to be fervent admirers of Adolf Hitler.