The gas pipelines. Gazprom is a key element of his kleptocratic rule. I don't think it's a coincidence that he waited so many years to "solve" the Donbass, but a few months after it became clear that NordStream2 will likely never become operational, suddenly he went balls-out. From a security standpoint he could have simply deployed in the key regions and dared anyone to fight back, like Erdogan has done over the Syrian regions he wanted to annex. Instead Putin is trying to take the whole thing, because he wants to control the pipelines all the way.