The main armament of these ships, the 12"/45 caliber Mark 5 gun, was also made available to the Army, but these were never deployed by the United States and at least some were sold to Brazil. Protection against air attack was slow to evolve. Existing batteries were camouflaged, but if detected, they remained vulnerable to air attack. The first batteries of heavy guns constructed after World War I were, somewhat inexplicably, completely open except for camouflage, but mounted long-range weapons set back from the coast out of direct observation from the sea. However, from the late 1930s (in most cases beginning in 1942) these batteries were mounted under thick concrete casemates covered with vegetation to make them virtually invisible from above and well protected against bombing. Significantly, the Washington Naval Treaty prohibited major improvements to defenses in the Pacific including the Philippines, so the two long-range 12-inch guns at Fort Mills on Corregidor were never casemated (paradoxically, this probably improved their usefulness against the Japanese invasion when it came, as they had large arcs of fire). Another result was that 12 240 mm howitzers being shipped to the Philippines were deployed in Hawaii instead. In anticipation of a conflict with Japan, most of the limited funds available between 1933 and 1938 were spent on the Pacific coast, especially as several Japanese aircraft carriers were operational by then. In 1939, the threat of war in Europe prompted larger appropriations and the resumption of work along the Atlantic coast. A new weapon adopted by the U.S. during World War I introduced road and cross-country mobility to the Coast Artillery. The 155 mm gun M1918 (6.1 inch), closely derived from the French 155 mm GPF (Grand Puissance Filloux, or high-powered gun designed by Filloux), could be towed behind heavy Holt tractors and deployed to provide some protection for areas not part of existing harbor defenses. Each tractor-drawn regiment was authorized 24 of these weapons. Circular concrete platforms called "Panama mounts" were constructed in existing and new defenses to improve the utility of these weapons, particularly early in World War II. In anticipation of war, additional mines, searchlights, radar, and anti-aircraft guns were installed in 1940 and 1941. However, due to a general shortage, installation of new anti-aircraft guns at harbor defenses was minimal. After the war began, the entire Western Defense Command was placed on high alert, but Japanese attacks, including two submarine deck gun attacks and explosive balloons, caused only minor damage.