So... "Trump fought in WW2 and was very helpful in securing it." - is something you'd understand as USA? Because typically we'd use the leader of the nation at that time, not currently.
Also, Trump seems to not understand that 80+ years have gone by and governments change, nor the whole cold war propaganda that he literally lived through.
Trump says to World Leader 2 "We lost half a million soldiers in WWII". World Leader 2 then says to someone else, referring to that conversation: "He lost half a million soldiers in WWII" and "He fought with us in WWII" Both of the later two sentences make perfect sense to consider Trump as the USA in this instance, considering that Trump is describing the USA as "we". Especially if during the conversation World Leader 2 clarified that "and he did fight, the US fought" to convey that "he" means the US.
They.
The correct pronoun to use when quoting a person using we, is they. We is a plural, in this case meaning the people of America. They, meaning the people of America in which you are not apart of, is how you'd say it.