The Kremlin’s launch of armed troops across an international border to attack Ukrainian defenders came four months after Russia’s near-bloodless takeover of Ukraine’s Crimean region.
From: Kyiv Post by Stefan Korshak

On the 23rd anniversary of Ukraine’s 1991 declaration of independence, in August 2014, uniformed Russian troops invaded in force, crossing Ukraine’s eastern border to prevent the defeat of Kremlin-sponsored “separatists” by Kyiv’s forces.
Ukrainian and even some international media identified and watched trucks carrying hundreds of paratroopers from Russia’s elite 76th Airborne Division rumble across the international frontier into Ukraine’s Donetsk region. Dark-green-painted tanks and infantry fighting vehicles marked with white invasion stripes led the columns.
Independent analysts later estimated that Russia sent around 10,000 heavily armed troops and advisors into Ukraine. That Russian invasion – albeit directly violating two treaties signed by Moscow with Kyiv to respect Ukraine’s borders – ground down Ukraine’s poorly equipped army.
Repeatedly, when Ukrainian defenses coalesced and separatist attacks stalled, the Kremlin committed more troops to break Kyiv’s forces. An epic siege of Donetsk airport, where Ukrainian paratroopers, mechanized infantrymen and special forces troops held out for nearly four months against near-daily assault, ended after Moscow sent elite Marine regiments from Murmansk to storm terminals or flatten them with artillery…