Sen. Arthur Vandenberg, the Republican chairman of the Senate Foreign Relations Committee in the late 1940s, is said to have declared that partisan politics must stop “at the water’s edge.” Few people remember Vandenberg, and it is not even clear if anyone ever took his ideal seriously, but it seems that fewer and fewer people even pay lip service to it. U.S. President Donald Trump seems to feel it’s perfectly appropriate to trash his political opponents while traveling abroad.

That’s disturbing enough. But the more consequential development in U.S. foreign policy—responsible for sowing instability and sapping American power—doesn’t involve partisanship abroad, but polarization at home.

It is now possible to divide the Middle East between “Republican Party countries” and “Democratic Party causes.” This is a phenomenon of at least the last decade, but it has become more pronounced during the Trump era. Aside from Jordan—there is almost no one in Washington who does not like King Abdullah—the lineups are clear: Israel, Egypt, the United Arab Emirates, and Saudi Arabia are Republican in the sense that the party’s leaders and voters are sympathetic to them or more sympathetic to them than Democrats are. At the same time, Democrats tend to identify with the Iran nuclear deal and the Palestinians.