As I understand the rules, Japan attacking UK/ANZAC before they move into China or declare war first gives the US the ability to declare war, and until then, the US can land onto Japanese spaces and move through Japanese waters without engaging Japanese forces. My friend landed men on Okinawa and Iwo Jima as the US before I attacked anyone as Japan, other than China. The next turn I attacked the UK, and my friend argued that then the US was entered into the war, and since it was my turn, I would enter combat on Okinawa and Iwo Jima as the attacker, and he would get the defense rolls. I argued that since I owned the land, he would be considered the attacker, and combat would begin on his turn, thus making him the attacker. We ended up doing it as me attacking, and I lost Iwo Jima.
How should that scenario have actually played out?