KYIV, Ukraine—Monday marked the one-year anniversary of Russia’s annexation of Crimea, a former Ukrainian territory.

Russia’s takeover of Crimea is an ongoing source of tension between Russia and Ukraine, and a potential flash point for escalating the nearly year-old war in eastern Ukraine. The United States and other NATO countries have labeled Russia’s move to take back the peninsula, which sits at the confluence of the Black Sea and the Sea of Azov, as illegal, and denounced a referendum voted upon last year to secede from Ukraine as illegitimate.

The Kremlin, however, has pushed back and claims the referendum reflects the will of the Crimean population. Crimea was originally Russian territory, but was ceded to the Ukrainian Soviet Socialist Republic in a deal by Soviet First Secretary Nikita Khrushchev in 1954.

Independent Ukraine retained control of Crimea following the 1991 breakup of the Soviet Union. Russia maintained a naval base, however, in the Crimean city of Sevastopol, which became Russia’s only warm water naval base.

The Daily Signal takes a closer look at the conflict over Crimea in these photos.

A mural showing the Crimean peninsula in Russian national colours on the wall of a building in Moscow's Taganskaya Square. The message reads 'Crimea and Russia are united forever!' (Photo: Zurab Dzhavakhadze/ITAR-TASS/ZUMAPRESS.com)

A mural showing the Crimean peninsula in Russian national colors on the wall of a building in Moscow’s Taganskaya Square. The message reads ‘Crimea and Russia are united forever!’ (Photo: Zurab Dzhavakhadze/ITAR-TASS/ZUMAPRESS.com/Newscom)

Supporters of the self-declared Donetsk People's Republic stand in front of the occupied regional administration building in Donetsk, Ukraine, May 12, 2014. The secession referendum on May 11 allowed the three million residents of the provinces of Luhansk and Donetsk vote on whether they want to remain part of Ukraine. (Photo: Maxim Shipenkov/Newscom)

Supporters of the self-declared Donetsk People’s Republic stand in front of the occupied regional administration building in Donetsk, Ukraine, May 12, 2014. The secession referendum on May 11, 2014, allowed the 3 million residents of the provinces of Luhansk and Donetsk vote on whether they want to remain part of Ukraine. (Photo: Maxim Shipenkov/Newscom)

Ukrainian soldiers drive on an Armored Personnel Carrier on a road near a military camp in Kramatorsk town, near Slaviansk, Ukraine, Sept. 4, 2014. NATO and Russia set themselves on a collision course as leaders attended a summit in Wales to ramp up support from the trans-Atlantic alliance in the fight against eastern separatists, provoking angry responses from Moscow. The summit was overshadowed by Russia's annexation of Crimea in March and five months of fighting between pro-Russian separatists and government forces in eastern Ukraine. (Photo: Roman Pilipey/Newscom)

Ukrainian soldiers drive on an armored personnel carrier on a road near a military camp in Kramatorsk town, near Slaviansk, Ukraine, Sept. 4, 2014. (Photo: Roman Pilipey/Newscom)

Ukrainian soldiers drive on an Armored Personnel Carrier on a road near a military camp in Kramatorsk town, near Slaviansk, Ukraine, Sept. 4, 2014. NATO and Russia set themselves on a collision course as leaders attended a summit in Wales to ramp up support from the trans-Atlantic alliance in the fight against eastern separatists, provoking angry responses from Moscow. The summit was overshadowed by Russia's annexation of Crimea in March and five months of fighting between pro-Russian separatists and government forces in eastern Ukraine. (Photo: Roman Pilipey/Newscom)

Peace talks have been complicated by Russia’s annexation of Crimea in March 2014 and a years of fighting between pro-Russian separatists and government forces in eastern Ukraine. (Photo: Roman Pilipey/Newscom)

Vehicles wait in a queue at Kavkaz port, on the eastern shore of the Strait of Kerch March 2, 2015. The ferry service links mainland Russia to Crimea. (Photo: Artyom Geodakyan/TASS/Newscom)

Vehicles wait in a queue at Kavkaz port, on the eastern shore of the Strait of Kerch, March 2, 2015. The ferry service links mainland Russia to Crimea. (Photo: Artyom Geodakyan/TASS/Newscom)

Sevastopol governor Sergei Menyailo looks on during a March 13, 2015 press conference on the annual results of his work in Crimea. (Photo: Yuri Mashkov/TASS/Newscom)

Sevastopol governor Sergei Menyailo looks on during a March 13, 2015, press conference on the annual results of his work in Crimea. (Photo: Yuri Mashkov/TASS/Newscom)

People take part in a rally in independence Square on March 13, 2015 in Kiev, Ukraine. (Photo: Str/NurPhoto/ZUMA Wire)

People take part in a rally in Independence Square on March 13, 2015, in Kyiv, Ukraine. (Photo: Str/NurPhoto/ZUMA Wire/Newscom)

A cossack attends a rally to mark  anniversary of controversial referendum to join Russia, in Simferopol, Crimea, March 16, 2015. (Photo: Artur Schvarts/Newscom)

A cossack attends a rally to mark anniversary of controversial referendum to join Russia, in Simferopol, Crimea, March 16, 2015. (Photo: Artur Schvarts/Newscom)

Crimean people spread a huge Russian flag with white words in it saying 'Russia Crimea Together Forever' and  inhabitants' inscriptions during a rally to mark  anniversary of controversial referendum to join Russia, in Simferopol, Crimea, March 16, 2015. (Photo: Artur Shvarts/Newscom)

Crimean people spread a huge Russian flag with white words saying ‘Russia Crimea Together Forever’ during a rally to mark anniversary of controversial referendum to join Russia, in Simferopol, Crimea, March 16, 2015. (Photo: Artur Shvarts/Newscom)

Russia's defense minister Sergei Shoigu holds a video link-up of Russia's defense ministry with Sevastopol as part of celebrations of the first anniversary of the 2014 Crimean status referendum on March 16, 2015. (Photo: Anton Novoderezhkin/TASS/Newscom)

Russia’s defense minister Sergei Shoigu holds a video link-up of Russia’s defense ministry with Sevastopol as part of celebrations of the first anniversary of the 2014 Crimean status referendum on March 16, 2015. (Photo: Anton Novoderezhkin/TASS/Newscom)

Posters on display at an exhibition titled ''Crimea. The return to its native harbor'' in Moscow's Novopushkinsky Park. The exhibition marks the first anniversary of the 2014 Crimean status referendum. (Photo: Sergei Fadeichev/TASS/Newscom)

Posters on display at an exhibition, ”Crimea: The return to its native harbor,” in Moscow’s Novopushkinsky Park. The exhibition marks the first anniversary of the 2014 Crimean status referendum. (Photo: Sergei Fadeichev/TASS/Newscom)

Parents and children gather outside a school in the Komsomolskoye village to protest the intention of local authorities to introduce education in Ukrainian. (Photo: Tass/Newscom)

Parents and children gather outside a school in the Komsomolskoye village to protest the intention of local authorities to introduce education in Ukrainian. (Photo: Tass/Newscom)

Clocks show the time in Sevastopol and Moscow on March 29, 2014. Crimea officially moved to the Moscow time zone one year ago. (Photo: Alexander Ryumin/Newscom)

Clocks show the time in Sevastopol and Moscow on March 29, 2014. Crimea officially moved to the Moscow time zone one year ago. (Photo: Alexander Ryumin/Newscom)

People holding Russian and Crimean flags as they gather at Lenin Square after the end of the referendum in Simferopol, Crimea March 16, 2014. (Photo: Yuri Kochetkov/Newscom)

People holding Russian and Crimean flags as they gather at Lenin Square after the end of the referendum in Simferopol, Crimea, March 16, 2014. (Photo: Yuri Kochetkov/Newscom)

Russian President Vladimir Putin with head of Crimean government Sergei Aksionov, chairman of Crimean parliament Vladimir Konstantinov, and head of Sevastopol city administration Alexei Chaliy sign the act of reunification Crimea with Russia in the Grand Kremlin Palace March 18, 2014. (Photo: EPA/Alexey Druzhinyn/Newscom)

Russian President Vladimir Putin with head of Crimean government Sergei Aksionov, chairman of Crimean parliament Vladimir Konstantinov, and head of Sevastopol city administration Alexei Chaliy sign the act of reunification Crimea with Russia in the Grand Kremlin Palace, March 18, 2014. (Photo: EPA/Alexey Druzhinyn/Newscom)

Local commuters walk along railway lines next to Ukrainian tanks ready to depart from Crimea to Ukraine March 31, 2014. (Photo: Yannis Behrakis/Newscom)

Local commuters walk along railway lines next to Ukrainian tanks ready to depart from Crimea to Ukraine, March 31, 2014. (Photo: Yannis Behrakis/Newscom)