That statement suggests that Iran will seek revenge against Israel in some other form. Iran may increase its support for Hamas or Palestinian Islamic Jihad. It will ensure that Israel remains “the lesser Satan” in Iranian propaganda for the foreseeable future, and Israeli soft targets — such as tourists and students — could be at risk, along with Israeli officials overseas. Americans, too, may be vulnerable for their association with Israel — on top of the Trump administration’s assassination of the Iranian senior general Qassim Suleimani in January.

With temperatures running so high, the incoming Biden administration now faces a serious challenge. Mr. Biden has vowed to return to negotiations with Iran, but he and his team cannot do much more than message through the media to Iran to stay patient until the inauguration on Jan. 20 — and to the Israelis to stop their campaign of sabotage.

Meanwhile, European countries that have diplomatic relations with Iran and are still parties to the nuclear agreement can help bridge the gap until the Biden inauguration. Britain, France and Germany should seek a swift convening of the commission that monitors implementation of the Iran nuclear agreement. Their foreign ministers should act even sooner and issue a statement condemning the assassination as illegal under international law and damaging to the cause of nonproliferation. A spokesperson for the European Union’s high representative for foreign and security policy has already described the killing as a “criminal act.”