A look at the conflict, its roots and possible outcomes: THE BREAKUP When the Soviet Union broke apart in 1991, parts of Ukraine were eager to escape the Russian leash and join with Western Europe and possibly NATO. But Russia's ties with the industrial heartland of eastern Ukraine run deep, with many Russian speakers living there. In addition, most people who live in Crimea are Russian-speakers, and Moscow has a naval base there where its Black Sea fleet is headquartered.
In 2013, the Ukrainian government was due to sign a deal supposed to open European Union markets for Ukrainian goods and put the country on a pathway to possible EU membership. Russia, Ukraine's closest neighbor and major trading partner, vehemently opposed the deal, fearing an uncontrolled flow of goods through what was then virtually an open border.
Then-Ukrainian President Viktor Yanukovych pledged to sign the deal — but walked out on it at the last moment. Mass protests followed in Kiev, decrying Yanukovych for what was seen as an attempt to deny Ukrainians a European future. A crackdown by riot police saw 130 people killed in sniper fire. Yanukovych fled to Russia.
ANNEXATION AND VIOLENCE Russia annexed the Crimea in 2014 after troops without insignia occupied crucial infrastructure, including Ukrainian military bases, while Ukrainian troops put up little resistance and retreated. It was only years later that Russian President Vladimir Putin publicly admitted that these were in fact Russian troops.
Activists in eastern Ukraine, backed by Russian operatives, took over towns and tore down Ukrainian flags. Separatist leaders in the cities of Donetsk and Luhansk held elections supporting a motion to break away from Ukraine.
Sporadic violence grew into a full-blown conflict in May 2014, when Ukraine launched an airstrike on the Donetsk airport that was overrun by Russian Chechen fighters. More than 10,000 people have been killed and over 1 million displaced, with large parts of the east still under separatist control. The Kremlin never admitted its role, portraying it as a civil conflict, but overwhelming evidence suggests Russia has been sending a sizeable number of troops and advisers as well as weapons to the rebels.
Ukraine signed peace accords with the separatists in 2015, calling for a cease-fire and political settlement in the east. While it helped to decrease the intensity of fighting, the accords did nothing to resolve the region's political stalemate.
ESCALATION AT SEA The latest area of rising tension occurred Sunday in the Kerch Strait, which links the Sea of Azov and the Black Sea off the eastern edge of the Crimean Peninsula. Russia has built a long bridge spanning the strait and connecting the peninsula with its mainland. In March, Ukraine detained a fishing vessel sailing from Crimea, and Russia increased its military presence in the area, inspecting all vessels sailing to or from Ukrainian ports, disrupting trade. Ukraine has protested, calling them an "economic blockade" affecting its ports in the industrial east.
Ukraine says two of its naval gunboats and a tugboat were fired on and seized by the Russian coast guard in the narrow strait. Russia says the Ukrainian vessels violated its territorial waters. Both sides blamed the other in the incident.
CHURCH CONFLICT In another sign of Ukraine's resolve to break with Russia, Kiev has stepped up efforts to seek independence for its Orthodox church. The church in Ukraine has been tied to the Moscow Patriarchate for hundreds of years, but calls for independence have increased since the conflict began.
The Istanbul-based patriarchate, whose head Bartholomew I is considered the "first among equals" of Orthodox church leaders, made the first step toward recognizing the independence of the Ukrainian Orthodox Church in October. The move was decried by the Russian Orthodox Church as well as the Kremlin.
GLOBAL IMPACT The ship seizure has prompted global concern and raised questions about eventual international intervention. NATO and the U.N. Security Council discussed what to do. Russia's actions are drawing renewed Western anger — and demonstrating Putin's resolve just days before he meets U.S. President Donald Trump at the Group of 20 summit in Argentina.
The European Union is calling on both sides to stay calm, but is distracted by negotiations on Britain's pending departure and divided over migration, and may have little energy to deal with a new crisis involving Russia and Ukraine.
WHAT'S NEXT? While Sunday's incident raises the specter of a full-blown conflict between Ukraine and Russia, it is not likely that either wants an all-out-confrontation, and the leaders of both countries might be getting an outcome that they want.
Putin, whose approval ratings have tumbled recently, reinforced the message that Crimea is Russian for good and that he won't allow anyone to question it. Ukrainian President Petro Poroshenko is likely to benefit from Ukraine's parliament voting to impose martial law in the country for 30 days. Poroshenko's approval ratings have been plunging, and playing up the Russian threat could help his chances of getting re-elected. He sought to allay fears that the presidential election would be postponed. After approving Poroshenko's motion on martial law, the lawmakers also voted to call the presidential election for March 31, in effect reaffirming the vote.