A cursory look at history (i.e. no deep dives needed) will reveal that there have been countless times in history when what would have seemed at the time (and indeed in hindsight) as unnatural verging on impossible.
Withing living memory, we have seen The British Empire join with the Soviet Union to oppose Hitler.
Going back, anyone familiar with the rise of Rome will know about the magic square of the Mediterranean and it's complex brew of ever shifting alliances that saw the fall of Carthage and the rise of Rome against the backdrop of Egypt and Greece.
Remember the British Empires mission statement: "Britain has no permanent allies and no permanent enemies; merely permanent interests". Which is not novel and not earth shattering; merely succinct. I would be amazed if such wisdom hadn't been nicked from ancient China. Possibly as a strapline to Sun Tzu. Although I could see Marcus Aurelius meditating on it. And even if he didn't say it, why would Plato not concur.
what will it take for this to actually happen?
What it always has. Dangerous, unpredictable times. Trump is acting like someone who has been given a double album, but so far has only played the title track which goes on about how the hero rose to power by defying all the rules. He hasn't yet turned it over* and heard the second side which details the heroes tragic demise as people band together to depose them. (Time spent reading classical mythology is never wasted).
*Under 40s - ask your parents