On the front lines in southeast Ukraine, soldiers make the rounds patrolling in the trenches, on alert for attack from Russia-backed separatists or Russian forces. This has been an active front line since 2015 … … after Russian separatists seized control of parts of eastern Ukraine. Now, soldiers wonder if they’ll soon be fighting against Russian military forces that have been mobilizing on the other side of the border. Fourteen miles from the front line is Mariupol, an industrial port city of about half a million people. In case of an invasion, this could be the scene of large-scale battles. It bears scars of rocket fire from separatists, and residents are now bracing for an assault that Russia denies is coming. We examined satellite images and videos posted to social media that showed they may have reason to fear. Our findings map out Russia’s military buildup on three sides of the Ukrainian border. To the north, Russia has moved heavy military equipment toward the border, some within striking distance of Ukraine’s capital, Kyiv, and Russia has deployed fighter jets to Belarus. To the south, Russia has moved more weapons and equipment to the Crimean peninsula, which it seized from Ukraine in 2014. To the east, in the Rostov-on-Don region, satellite imagery shows a recent expansion of tents for housing at one of Russia’s biggest military bases. Videos posted by the Russian Ministry of Defense also show live-fire drills. Nearby is the Sea of Azov, which has shared waters. Russia has stepped up its naval presence here, and it could be the site of a new attack or provocation, and that’s put the Ukrainian Maritime Guard on edge. They often patrol these waters within eyesight of Russian vessels, which outnumber Ukrainian ships four to one. After years of conflict, some city residents are ambivalent about the risk of escalation. Others are preparing for the worst, even a city official. Mikhail Vershinin is a former volunteer fighter who now heads the Donetsk regional patrol police. But many of the people in this city have ties to Russia, and fear the consequences of Ukraine standing up to President Putin. Some, including soldiers, asked to withhold their last names out of concerns for their personal safety. Just east of Mariupol, the town of Shyrokyne is a stark reminder of the cost of war. All that remains is a band of Ukrainian soldiers dug in among the ruins. One village north, Zoya Kralya still tends to her plot and animals. She’s one of the few remaining residents. Her daughter moved to Russia after the conflict began, and she now lives alone. After years of battling separatists, Ukrainians here know what a Russian invasion could mean … … and just how much could be lost.