Very deeply buried shelters provided the most protection against a direct hit. The government did not build them for large populations before the war because of cost, time to build, and fears that their very safety would cause occupants to refuse to leave to return to work, or that anti-war sentiment would develop in large groups. The government saw the Communist Party's leading role in advocating for building deep shelters as an attempt to damage civilian morale, especially after the Molotov-Ribbentrop Pact of August 1939.:34