The United States issued an embargo because of the fact that Japan invaded Manchuria. . They were increasingly violent and they were trying to conquer the region, basically. This was the boiling point that led to the attack on Pearl Harbor. With the embargo in place, Japan was unable to trade properly or have necessary essential like oil. This embargo is the reason why General Tojo planned to bomb Pearl Harbor. It is in response to Pearl Harbor, that America declared war and fought in world war II.