After routing Russian forces in the country’s northeast, Ukraine has to convert its initiative quickly into a defensible position—or total victory.

(Originally published Sept. 13 in “What in the World“) As Ukraine’s surprise attack in the north routs Russian occupation troops to liberate eastern Kharkiv Oblast, hopes are rising for Ukrainian victory and even for the downfall of Russian President Vladimir Putin as his own hardliners and elites take the latest retreat as a sign of his own failure. Ukraine’s supporters in Europe are saying Russia’s retreat justifies the economic sacrifices Europe is making, notably in terms of energy costs, and are calling for even more military aid.

While the Russian retreat is hopefully a turning point that marks the beginning of the war’s end, most wars don’t proceed in a straight line. We may thus be entering a potentially more dangerous phase of the war, in which Putin becomes cornered militarily and politically. Ukraine’s smaller force will have to press their attack without being surrounded and then hold territory they’ve liberated as cold weather sets in.

And while winning back breakaway provinces in Donbas might seem the perfect bargaining chip for a negotiated peace, Ukrainian President Volodymyr Zelensky has made clear he will settle for nothing less than the complete restoration of Ukrainian territory. It’s not clear that Putin is yet defeated, or that he won’t resort to more drastic tactics—even tactical nuclear weapons—to avoid defeat. And as Anne Applebaum points out in her article in The Atlantic, Putin has no clear successor, or a viable opponent waiting in the wings to take the reins in a stable political transition.

North Korea, which is now supplying Russia with ammunition for the war, has passed a law allowing pre-emptive nuclear strikes as the hermit nation prepares to conduct its first nuclear weapons tests since 2017.