Some external group doing a terrorist attack? Nah, they don’t need to. The US can attack themselves easily and will. With the coming high court ruling coming about Trump being withheld from voting slips (correct term? Not being alowed to be on the list during election) will be a huge thing. Either the very conservative judges he appointed will rule he incited a riot and should not be allowed in office again (according to the intention of article 14 of the US constitution, which those judges say they adhere), or they will say it’s not up to us.
In the 1st case, the Trump adepts will start the 2nd civil war, in the 2nd case, who knows. Leaving it to the states could result in problems in some.
However you look at it, the USA is on it’s way to a massive implosion. With a destabalised political system like that, which state needs enemies. No real democratic system has only 2 parties.