There are no international treaties or laws preventing a military deployment now that Mr. Putin has invaded Ukraine. The democratically elected government of Volodymyr Zelensky would welcome such a troop presence. Mr. Putin believes he is dictating events, and so far Western nations are going along. That dynamic should be reversed.

Western powers have dispatched thousands of troops to frontline NATO states for humanitarian purposes. But these forces’ impact would be far more consequential inside Ukraine. Western powers should insert heavily armored forces into pockets of western Ukraine, making clear that such deployments are at the invitation of the sovereign government, are designed to safeguard humanitarian operations, and won’t engage offensively with Russian forces.

Such forces, drawn from NATO states and possibly other allied countries, could be structured similarly to NATO-led missions in Kosovo and Afghanistan. If Mr. Putin’s ground forces remain bogged down in eastern Ukraine, this humanitarian-oriented force could gradually move east. The recent uptick in airstrikes in parts of western Ukraine underscores the need to deter Mr. Putin bringing his indiscriminate bombing campaign to towns and cities on NATO’s periphery.