To waive or not to waive. President Trump again faces that question Thursday, a legislatively mandated deadline to waive or reimpose sanctions on Iran. The protests that have shaken the Islamic Republic over the past two weeks have invested this deadline with particular significance. They have raised the stakes and sharpened the debate.

On one side are those who advocate reimposing the sanctions—in effect scrapping the nuclear deal entirely. The arguments for this approach are familiar. The deal puts Iran on a glide path to...