Very few analyses of the Ukraine situation consider water to be very important, yet Ukraine's cutting off water supplies to Crimea combined with the recent droughts the area has seen is likely among Putin's most significant motivations. It might be that this invasion was in the works as early as 2016 and that Putin wanted "crazy man Trump" out of the way before executing it. That is, after all, about the length of time Crimea has been in a water crisis.
Of course, one could doubt the seriousness of Crimea's water crisis: However, the amount of land under cultivation in Crimea went from 130,000 hectares to 14,000 hectares since Putin seized it---and many areas have water available only three to five hours a day. Having grown up in drought prone California, I can attest that is one hell of a drought.
This whole situation may have had nothing to do with any actions the US government has taken after the Maidan revolution at all as Putin likely decided upon this course of action when the water crisis became obvious; it may not have as much to do with revanchism as I initially believed. There is essentially a straight line connecting the Obama administration's support for Yanukovych's removal, to the annexation of Crimea, to the cutting off of the water supply, and finally to this invasion. No intermediate steps are required. The unrest in the Donbas and NATO expansion may have played a role but they are not necessary to explain Putin's actions---and neither is Putin's revanchism despite my belief that it remains a motivating factor. Indeed, the restriction of water supplies explains why Putin is so inclined to exaggerate the "Nazi angle." In his mind, Nazism and siege warfare go hand in hand---limiting water supplies makes his mind think back on Leningrad.
Water in terms of access to a year-round port motivated Putin's initial seizure of Crimea, and water for irrigation and drinking is at the heart of the present invasion. Putin is a geopolitical thinker, and water is of the outmost importance to him.
[An interesting side note: I wonder if Donbas is pro-Russian because it has been, historically, a coal mining area; the Soviets likely treated coal miners, "true proletarians,'' better than the "mere peasant farmers" that make up the rest of Ukraine. This superior treatment may have led them to feel differently about "mother Russia."]