The simple answer to "why now" is that Russia isn't choosing their own time. Certainly Russia is acting as a spoiler in both Syria and the Ukraine, but in both cases the Russian involvement is reactive, not proactive. The Russians didn't initiate the Syrian crisis: the 2011 attempt to expand the Arab Spring into Syria was not their doing. They reacted when one of their very few external allies was threatened. The Russians didn't initiate the Maidan revolt either: they reacted to the perceived threat of having a border state that they've long considered part of their natural sphere of influence move firmly into the Western orbit.
Conspiracy theories aside, these situations grew out of local conditions that were not created (or in many ways fully anticipated) by the US, by "the West" collectively, or by Russia. The external players are in reactive roles, trying to turn events to their advantage. "Why now" was not determined by a decision from any outside player, the outside players simply responded to local events.
It may be true that Russian confidence and relative capacity is growing, but Russian influence is not. If the Ukraine emerges from this with a firmly pro-western regime (with or without Crimea and Donetsk), Putin's only ally on his western border will be the consummate loose cannon that is Lukashenko. Sooner or later he will fall (they all do) and who knows what happens then? Assad may well remain in power, but Syria will be a broken state and as much liability as asset for Putin for years to come. Chinese influence is growing in the 'Stans. Worldwide, nations that find themselves out of favor in the West are increasingly looking to China, rather than Russia, for support and leadership. It's difficult to argue that Russia is in any way ascendant in the global influence derby. Who do they influence?
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