A history of the tensions between Ukraine and Russia.

Two former republics of the Soviet Union — Russia and Ukraine — are once again in conflict. Here are some pivotal moments in the years leading up to Russia’s invasion of Ukraine on Feb. 24, as well as a brief look at their relationship in the 20th century.

February 2014 — Protesters in Ukraine overthrow President Viktor Yanukovych, who was friendly to Russia’s interests. During the revolution, more than 100 people are killed in protests that centered on the main square in the capital Kyiv, often called the Maidan.

The interim government that follows this pro-Western revolution eventually signs a trade agreement with the European Union that is seen as a first step toward membership in the bloc.

April 2014 — Russia invades and then annexes the Crimean Peninsula. Secessionists in eastern Ukraine, backed by Russia, declare themselves independent, as the Donetsk People’s Republic and Luhansk People’s Republic, and go to war against Ukraine.

The secessionist war continues in the eastern region known as Donbas. It then spreads west. Roughly 13,000 Ukrainian soldiers and civilians eventually die in the conflict. The front lines have barely shifted for years.

2014 and 2015 — Russia, Ukraine, France and Germany sign a series of cease-fire agreements known as the Minsk Accords. Many view these accords as ambiguous.

April 2019 — A former comedian, Volodymyr Zelensky, is elected by a large majority as president of Ukraine on a promise to make peace with Russia and restore Donbas to the country.

2021-2022 — President Vladimir V. Putin of Russia seeks to prevent Ukraine’s drift toward the United States and its allies. Mr. Putin demands “security guarantees,” including an assurance by NATO that Ukraine will never join the group and that the alliance pulls back troops stationed in countries that joined after 1997.

Many Russians view the Ukrainian capital, Kyiv, as the birthplace of their nation and cite the numerous cultural ties between the two countries.

Here is a brief recap of their relations in the 20th century:

1918 — Ukraine declares independence from Russia during a conflict fought by multiple countries and armies over several years. Its independence and sovereignty receive international recognition at the Treaty of Brest-Litovsk. Soviet forces later overthrow independent Ukraine. The Ukrainian Soviet Socialist Republic is founded in 1921, and Ukraine is subsumed into the Soviet Union the following year.

1932 and 1933 — A famine caused by Stalin’s policy of collectivization kills millions of people, mainly ethnic Ukrainians in a republic that is known as the bread basket of the Soviet Union. The disaster is known as the Holodomor, from the Ukrainian word for famine.

1939-1944 — The Soviet Union annexes what is now western Ukraine from Poland and Romania. Later, Nazi Germany and the Axis powers invade the Soviet Union and occupy Ukraine, which suffers enormous devastation.

1991 — Ukraine declares independence, a move endorsed in a referendum by 92 percent of voters. Russia, Ukraine and Belarus sign an accord recognizing that the Soviet Union has dissolved. Ukraine begins a transition to a market economy, and comes into possession of a significant stockpile of nuclear weapons that had belonged to the Soviet Union.

1994 — Under the Budapest Memorandum, Ukraine gives up its nuclear arsenal in exchange for a commitment from Moscow “to respect the independence and sovereignty and the existing borders of Ukraine.”