But he couldn't stop, for several reasons, the first one being his ultimate goal was to reverse the outcome of the First World War, and restore Germany to its position in, say, March 1918:
A second reason, perhaps even more important, as Victor Davis Hanson points out in this audio: imperial conquest is not only fun, it's addictive.
Once hooked on it, leaders find they can't live without the adulation and glorification it brings them.
And so they continue, until the battlefield results turn against them, and they are forced (often by death) to "kick the habit".
From the international perspective, our goal must be to make the taste of "victory" so bitter, leaders like Putin will not wish to eat of it again.