When one side feels it has a substantial advantage either in numerical superiority or technological advancement that it believes other side can't or won't counter.
Remember reading of China-Russia border conflict where China felt grabbing a bit of territory and Russia wouldn't react.
Russia brought up Artillery and bombed the hell out of the area on Chinese side that made whole area a wasteland and made clear it had no intentions of stopping.
China realised it had overstepped the mark and that Russia was not going to play ball.