One, by supposedly Zionist left wing Jews, was that while ultimately part of Jerusalem would be Israel's capital, now was not the right time to make such a declaration.
Two, often made by diplomats and Middle East "experts," was that the move would make Arabs very, very, very angry and would ignite the region into a huge terror war.
Three, made by the Palestinians and their Arab allies to the West, was that such a move would destroy any chance for peace.
It's funny, because those same arguments are used all the time to stop anything happening that Palestinian Arabs (or Arabs altogether) don't want.
