In the early days of the Ukraine invasion, one of the main lines pushed by Putin’s defenders was that the expansion of NATO posed a threat to Russia and that Ukraine was about to join. This didn’t stand up to even momentary scrutiny. The Baltic States had been members since 2004 without doing anything to threaten Russia.
And while Ukraine’s constitution included a goal of joining NATO, Zelenskiy was describing this as a ‘remote dream’ even before the invasion took place, and clearly indicated willingness to abandon the idea in return for peace.
But there is an important sense in which NATO shares responsibility for this disaster. The US intervention in Kosovo, including the bombing of Belgrade, was undertaken by NATO, to avoid the need to get the support of the UN Security Council, where Russia had a veto. This was a substantial breach of international law, followed by a much bigger breach in the invasion of Iraq.