Isn't the capital of Israel what Israel says it is?
I know the default thinking here is Trump is automatically assumed to be doing something something terrible but...
Israel does say Jerusalem is their capital, it's everyone around Israel that says it shouldn't be.
This move is controversial not because Trump is trying to force Israel into something, it's because Jerusalem is a contested piece of land with religious significance to the Palestinians as well.
Now, I'm not saying this isn't a bad move, but the assumed notion some people are making is this is bad move because it's unfair to Israel, when that isn't the case or intention.
Basically Muslims and The Jews have been fighting over their holy land for thousands of years and no one can seem to agree on who has rights over this land.