The shortest driving Kiev to Moscow is 532 miles (includes toll).
Other cities will be well within range. Rostov for instance.
This will be the straw.
Ukraine is not allowed to attack russian cities with western weapons-Himars, Storm Shadow, etc.
US says Ukraine will not use US-supplied rocket systems to hit Russian territory-June 2022
https://www.theguardian.com/world/2022/jun/01/us-says-ukraine-will-not-use-us-supplied-rocket-systems-to-hit-russian-territory