He even envisioned a grand summit to announce a peace deal. But Putin rejected repeated American calls to stop his attacks. And Trump, in recent months, began to take that personally, complaining privately to advisers—and then eventually in public—that Putin would tell him one thing in their phone calls (that he was committed to peace) and then act entirely differently afterward (by bombing Ukraine).

For President Vladimir V. Putin of Russia, it is an opportunity not just to end the Ukraine war on his terms, but to split apart the Western security alliance.