The M3 37mm Anti-Tank Gun was the primary gun for U.S. forces when the United States entered WWII.
The 37mm first saw action against the Japanese in the Philippines in December, 1941. In October 1942, the USMC used them with great success against Japanese armored units in Guadalcanal.
In Europe, by mid-1943, the 37mm gun was completely obsolete against German armor. But, in the Pacific, the 37mm was still found to be effective against weaker Japanese armor and continued in service there until the end of the war.