Every year for a number of years, Russia has run a substantial military exercise on the border of Ukraine. Putin is wary of overcommitting, but Russian decision to declare victory and pull troops from Syria, enables a more aggressive "exercise" this summer.
Europe hasn't welcomed Ukraine into NATO (or even the economic European Union) and is dependent on Russian oil/gas. US sanctions haven't been adequate to drive a Russian decision to pull forces from Donbass or Crimea, Ukraine. Without "boots on the ground," Russian forces will be capable of overwhelming Ukraine resistance. Here is a graphic showing how the US can help: this approach provides US Reserve and Active forces, minimizing a commitment while adding substantial "teeth" to Ukraine forces, which are also arrayed across the country (although not in sufficient number to stand up to Russian forces we see participating in the annual Zapad exercises).
Materials on the 2018 Zapad exercise haven't emerged yet, but details on last year's Russian exercise are here. Placing units in Ukraine before a Russian occupation will stop such an occupation without bloodshed, while waiting until after a Russian occupation cedes the country to Russia. Decisive action makes any leader popular: Putin is running for reelection in March 2018.