The simmering conflict in Eastern Ukraine has threatened to boil over the past few days, with Russia massing troops near the border and pro-Kremlin media raising the specter of war.

Why it matters: U.S. European Command went into high alert in light of the Russian movements, which some experts speculated could presage an active Russian military intervention in the Donbas region of Eastern Ukraine, where Ukraine has been fighting pro-Russian separatists for seven years.

The backstory: Kremlin hopes that Ukrainian President Volodomyr Zelensky would cut a peace deal on Russia's terms have faded, leaving a diplomatic deadlock.

• Direct intervention in the Donbas could give Putin leverage, bolster his standing ahead of parliamentary elections this year, and potentially help secure a much-needed water source for occupied Crimea.
• But the costs of such a war likely exceed any benefits for Moscow, removing all plausible deniability from Russia's war in Ukraine and risking a strong Western response.

The other side: Zelensky tweeted Thursday that he was heading to the Donbas because a Ukrainian soldier had been killed overnight and he wanted "to be with our soldiers in the tough times."

• The crisis has sparked a burst of patriot fervor and a strong show of support from Washington, both of which have strengthened the embattled Ukrainian president, notes Dmitri Trenin of Carnegie Moscow.
• President Biden and virtually his entire top team called their Ukrainian counterparts over the past week, and NATO Secretary-General Jens Stoltenberg called Zelensky on Tuesday.
• While Kiev was dismayed when the French and German leaders met with Putin (and not Zelensky) during the crisis, German Chancellor Angela Merkel Thursday "demanded" in a call with Putin that "this build-up be unwound in order to de-escalate the situation," her office said.

What to watch: Russian officials and pundits have been warning that Russia's hand might be forced if separatists in Donbas are slaughtered by Ukrainian troops. There's no clear factual basis for such fears, but they could provide a pretext for intervention.

The bottom line: The flare-up is a reminder that the war in Ukraine is far from over and could deepen dramatically, now or some day down the line.