In 1940 and early 1941, Hungary, Romania, and Bulgaria all agreed to adhere to the Tripartite Pact and thus join the Axis. Hitler then pressured Yugoslavia to join as well. The Regent, Prince Paul, yielded to this pressure and declared Yugoslavia's accession to the Pact on 25 March 1941. This move was highly unpopular with the Serb-dominated officer corps of the military and some segments of the public: a large part of the Serbian population, as well as liberals and Communists. Military officers (mainly Serbs) executed a coup d'état on 27 March 1941, and forced the Regent to resign, while King Peter II, though only 17, was declared of age. Upon hearing news of the coup in Yugoslavia, on 27 March Hitler issued a directive, which called for Yugoslavia to be treated as a hostile state. The Germans started an invasion with air assault on Belgrade on 6 April. The same day, Italians started the bombardment of Mostar that lasted for several days, damaging buildings and the Catholic church.