The bottom line is that Trump decided to announce that his administration is officially recognising Jerusalem as the capital of Israel for only one reason - because he promised his supporters that if he was elected, he would do so. That's it. He didn't do it as a favour to Israel. It wasn't something that had been keeping him up at night. Plainly and simply, he did it to appease his home base.
That being said, the why is not as important a question as "What happens now?" Well, for starters, the announcement and the decision not to back down from it will definitely cause tension between the US and its allies. In essence, it wasn't a very smart thing for Trump to have done in the first place. American recognition of Jerusalem as the legitimate Israeli capital doesn't have much of an effect on the situation. If anything, it will only cause further tension and strife. Had Trump reneged on his promise, it's unlikely that his supporters would have been that up in arms about it, especially if he had wisely chosen to fulfill a promise that didn't carry such obvious weight and invite such inherent headaches.
What difference will it make if the United States stands alone in this decision? To achieve real legitimacy, the world community at large would have to concur with the United States, but it's obvious that any sovereign state that does is in the minority. The wide majority of independent nations will continue to keep their consulates and embassies elsewhere. The announcement will infuriate the Arab world, elate hard-liner Jews and mean nothing to everyone else, except that it will demonstrate that the United States is fervent in its blatant favoritism of the Jewish state and that any attempt at real impartial mediation will have to come from someone else.
In time, things will go back to normal. Both sides will reinstate the status quo and the entire incident will fade from our memories. As years go by, other countries will refuse to recognise Jerusalem and the US decision to relocate its embassy and personnel there will come back to haunt them as the complex will inevitably be plagued by protests and will remain under constant threat.
It's certainly not putting America First to make poor decisions that paint the country in a bad light internationally.