When the president of Ukraine, Viktor Yanukovych, fled the country after the killing of 100 peaceful protesters on the Maidan in Kiev, Ukraine, he realized he had overstepped the bounds, and he had nowhere else to go but to flee to Russia.
At that point, Russia implemented their plan because they knew a new western-focused power would take over in Ukraine. They realized they couldn't control the political movements and the leadership any longer, and that's why they decided to destabilize the country.
At that point, when Ukraine was at its weakest—they didn't have a president, but had an acting president; they had no military—it essentially allowed the Russian-led operatives, because these were all—and this is public information, who these people were. They were Russian citizens, former Russian intelligence and army officers—they were the ones who led these so-called Donetsk People's Republic and Luhansk People's Republic. They led those aggressive, forceful, military takeovers of those parliaments.