The fight for the General Post Office, conducted simultaneously with that for City Hall, was especially difficult because of the construction of the building and the nature of the interior defenses. For three days XIV Corps and 37th Division artillery (and 4.2-inch mortars) pounded the Post Office. But each time troops of the 1st Battalion, 145th Infantry, attempted to enter, the Japanese drove them out. Finally, on the morning of the 22 February, elements of the 1st Battalion gained a secure foothold, entering through a second story window. The Japanese who were still alive soon retreated into the basement, where the 145th Infantry troops finished off organized resistance on the 23rd.