It was April 2022, and the diplomatic delegations from Ukraine and Russia were meeting in Istanbul just weeks after the Russian invasion of Ukraine had begun. Although there were details—especially on the size of the Ukrainian armed forces after the war and on the nature of security guarantees for Ukraine—to be worked out, a draft agreement had been signed by both sides. According to the Ukrainian delegation, Putin “demonstrated a genuine effort to find a realistic compromise and achieve peace.” Oleksandr Chalyi, a member of the Ukrainian negotiating team, said, “We managed to find a very real compromise. We were...