The conflict in eastern Ukraine began in April 2014 with low-level fighting between the Ukrainian military and Russian-backed separatist rebels who seized some towns in predominantly Russian-speaking eastern Ukraine. It has since escalated to outright-if-undeclared war between Russia and Ukraine. Separatist rebels began popping up in eastern Ukraine shortly after Russia had invaded and annexed Crimea, where supposed Crimean separatists actually turned out to be unmarked Russian special forces. They seized towns like Sloviansk and Donetsk, in the eastern region known as Donbas, ostensibly in outrage against the protests that had toppled Ukraine's pro-Russia President Viktor Yanukovych, himself from that same eastern region. That feeling of disenfranchisement among eastern Ukrainians is real, and the rebels likely do have some organic, local support. Still, the rebels were armed and supported by Russia's government. One of the most important rebel leaders, Igor "Strelkov" Girkin, is a Russian citizen and military veteran who retired from Russia's internal security services just weeks before he began leading the rebels, who are widely thought to include unmarked Russian special forces. Thousands of Russian troops massed on the border just across from the rebellion. For months, Ukraine did not move very aggressively against the rebels: Putin had not-so-subtly hinted that, if they did, he would blame the Ukrainian government for any deaths and invade to protect the Russian-speaking citizens of eastern Ukraine, whom he implicitly considers to be more Russian than Ukrainian. Everyone wanted to negotiate a peace deal. Things got bad in early July, when the Ukrainian government launched an offensive to push out the rebels once and for all. Russia started arming the rebels with high-tech surface-to-air missiles; on July 17 a civilian airliner with 298 people on board was shot down over eastern Ukraine, most likely accidentally by the rebels , and the world had finally had enough. Ukraine redoubled its offensive, the rebels looked on the verge of getting overrun, and in mid-August Russia escalated from covertly supporting the rebels to overtly invading with Russian military troops. Russia denies it is invading, but the evidence is overwhelming: on August 15, Ukraine said its military engaged Russian tanks crossing the border; on August 16, the leader of the Russian-backed rebels announced he had received 1,200 troops from Russia; on August 21, satellite imagery spotted Russian artillery crossing the border to fire at Ukrainian forces; on August 26, someone in Ukraine took a video of Russian tanks crashing through town, and so on. The Russian military is one of the largest in the world and is pushing back the Ukrainian forces fairly quickly. It is not clear whether they plan to occupy and annex eastern Ukraine as they did in Crimea, or simply to defend the rebels against getting overrun — if Putin even has a strategy at all. The US and Europe, while clearly outraged and punishing Russia with economic sanctions, have no plan to intervene, as putting Western troops into direct combat with Russian troops could make the risk of World War Three too great. So there is nothing stopping the Russian tanks.