Pereislav Agreement - An alliance between Ukraine and Russia against Poland. This then led to Ukraine being divided between Russia and Poland in the Truce of Andrusovo.
Ukrainian Government Abolished - Catherine the Great, ruler of the Russian Empire, abolishes the already heavily weakened Cossack Hetmanate, which was the somewhat independent government of Ukraine.
Ukrainian-Soviet War - Near the end of WWI, Ukraine gained independence due to the empire falling apart. However, when the Soviets began taking over Russia, they also targeted Ukraine and attempted to incorporate their territory into the Soviet Union. By the end, Ukraine was split between the USSR and a few other nations.
Holodomor - A genocide on the Ukrainian people performed by the USSR in order to eliminate Ukrainian resistance to the regime. This was done by enforcing unrealistically high quotas for crop production and making it a crime for a person to take any of those crops to feed themselves. The total death count was over 4 million.
Ukrainian Insurgent Army (UPA) attempts to gain independence - When most of Ukraine was under the control of Nazi Germany, roughly half a million Ukrainians attempted to fight back and not just stop the Nazis from invading further, but also to gain independence for Ukraine.
Ukraine gains independence once more - After the fall of the Soviet Union, Ukraine became an independent nation, and has remained as such to this day. This has been the longest period of independence for Ukraine in a very long time.