U.S. Army Soldiers were always armed with the M1 garand. However, despite being the late 1950's, they appeared to be dressed in 1940's era uniforms and gear.
That was the first game in the Destroy All Humans! series that featured Military Personnel from other countries.
Soldiers were initially only armed with the M3 Sub-machine gun (except the British Soldiers who always carried them) When the Alert Level was raised, Rifles, (Machine guns in Japan's case) and Anti-Armor weapons were brought to the fight. The Americans had M16A1s, Soviets have AK-47s, and the Japanese had the outdated Type-99 Light Machine Gun. U.S Army Officers had a Beret, and always carried a M1917 revolver.