He has been encouraged to do so. Neville Chamberlain made the exact same comment about Hitler invading Poland in 1938. It's appalling that nobody ever sees history repeat itself before it does.
In 1938, nobody had weapons in quantities that could end human civilization in less than 45 minutes. Putin has already said implicitly that nuclear weapons are on the table. No country in NATO is going to take that risk for Ukraine, so for now (to avoid a larger war) we'll just have to let Ukraine take the brunt of it.
If Putin's forces cross the border into Poland, then nice knowing you folks, because all bets are off.