Russia invaded Ukraine's Crimean Peninsula in February 2014 amid the deadliest days of the EuroMaidan Revolution that eventually ousted pro-Russian ex-President Viktor Yanukovych. Around 30,000 Russian troops crossed into Crimea, taking hold of the peninsula by early March 2014.
This event effectively began Russia's ongoing war against Ukraine. The Russian military has continued to occupy Crimea ever since. Many countries have openly opposed the illegal annexation of Crimea and continue to recognize Crimea as being part of Ukraine.
Crimea covers an area of around 27,000 square kilometers (10,400 square miles), which is roughly the same size as the U.S. state of Massachusetts.