Avigdor Lieberman’s previous stints as Israel’s foreign minister under Prime Minister Benjamin Netanyahu were a disaster for Israeli-American relations. Mr. Lieberman’s ultranationalist positions on Palestinians, settlements and the Israeli-Palestinian conflict rendered him effectively unwelcome in Washington and toxic to Palestinians. Yet to shore up his coalition in the Israeli Parliament, Mr. Netanyahu has now offered Mr. Lieberman the office of minister of defense — widely considered to be the second most powerful position in the Israeli government, with a critical role in dealing with the United States and the Palestinians.

Mr. Netanyahu may think his political needs are more important than relations with the soon-to-end Obama administration, relations that are already severely strained by the nuclear agreement with Iran. But the administration had at least established a working relationship with Moshe Yaalon, the tough but pragmatic defense minister who resigned once the offer to Mr. Lieberman became known. The timing of this changing of the guard is particularly sensitive because a critical 10-year defense agreement establishing new levels of American military aid for Israel is in the final stages of negotiations.

It’s hard to imagine peace talks moving rapidly forward in the immediate future, for a number of reasons. But it is entirely possible to imagine Israel’s relations in the region and beyond moving backward with a defense minister who has threatened, among other things, to conquer Gaza or bomb the Aswan Dam in the event of a war with Egypt. Mr. Lieberman’s ties with Israel’s own military establishment are frayed, most recently by his defense of an Israeli soldier arrested for executing a wounded Palestinian.