This article originally appeared at Brookings.

March 18 marks the fifth anniversary of Russia’s illegal annexation of Crimea, which capped the most blatant land grab in Europe since World War II. While the simmering conflict in Donbas now dominates the headlines, it is possible to see a path to resolution there. It is much more difficult with Crimea, which will remain a problem between Kyiv and Moscow, and between the West and Russia, for years—if not decades—to come.

THE TAKING OF CRIMEA

In late February 2014, just days after the end of the Maidan Revolution and Victor Yanukovych’s flight from Kyiv, “little green men”—a term coined by Ukrainians—began seizing key facilities on the Crimean peninsula. The little green men were clearly professional soldiers by their bearing, carried Russian weapons, and wore Russian combat fatigues, but they had no identifying insignia. Vladimir Putin originally denied they were Russian soldiers; that April, he confirmed they were.

By early March, the Russian military had control of Crimea. Crimean authorities then proposed a referendum, which was held on March 16. It proved an illegitimate sham. To begin with, the referendum was illegal under Ukrainian law. Moreover, it offered voters two choices: to join Russia, or to restore Crimea’s 1992 constitution, which would have entailed significantly greater autonomy from Kyiv. Those on the peninsula who favored Crimea remaining a part of Ukraine under the current constitutional arrangements found no box to check.

The referendum unsurprisingly produced a Soviet-style result: 97 percent allegedly voted to join Russia with a turnout of 83 percent. A true referendum, fairly conducted, might have shown a significant number of Crimean voters in favor of joining Russia. Some 60 percent were ethnic Russians, and many might have concluded their economic situation would be better as a part Russia.

It was not, however, a fair referendum. It was conducted in polling places under armed guard, with no credible international observers, and with Russian journalists reporting that they had been allowed to vote. Two months later, a member of Putin’s Human Rights Council let slip that turnout had been more like 30 percent, with only half voting to join Russia.

Regardless, Moscow wasted no time. Crimean and Russian officials signed a “treaty of accession” just two days later, on March 18. Spurred by a fiery Putin speech, ratification by Russia’s rubberstamp Federation Assembly and Federation Council was finished by March 21.

ATTEMPTS TO JUSTIFY

Moscow’s actions violated the agreement among the post-Soviet states in 1991 to accept the then-existing republic borders. Those actions also violated commitments to respect Ukraine’s sovereignty, territorial integrity, and independence that Russia made in the 1994 Budapest Memorandum on Security Assurances for Ukraine and 1997 Ukrainian-Russian Treaty of Friendship, Cooperation and Partnership.

In late March 2014, Russia had to use its veto to block a U.N. Security Council resolution that, among other things, expressed support for Ukraine’s territorial integrity (there were 13 yes votes and one abstention). The Russians could not, however, veto a resolution in the U.N. General Assembly. It passed 100-11, affirming Ukraine’s territorial integrity and terming the Crimean referendum invalid.

Russian officials sought to justify the referendum as an act of self-determination. It was not an easy argument for the Kremlin to make, given the history of the two bloody wars that Russia waged in the 1990s and early 2000s to prevent Chechnya from exercising a right of self-determination.

Russian officials also cited Western recognition of Kosovo as justification. But that did not provide a particularly good model. Serbia subjected hundreds of thousands of Kosovar Albanians to ethnic-cleansing in 1999; by contrast, no ethnic-cleansing occurred in Crimea. Kosovo negotiated with Serbia to reach an amicable separation for years before declaring independence unilaterally. There were no negotiations with Kyiv over Crimea’s fate, and it took less than a month from the appearance of the little green men to Crimea’s annexation.

The military seizure of Crimea provoked a storm of criticism. The United States and European Union applied visa and financial sanctions, as well as prohibited their ships and aircraft from traveling to Crimea without Ukrainian permission. Those sanctions were minor, however, compared to those applied on Russia after it launched a proxy conflict in Donbas in April 2014, and particularly after a Russian-provided surface-to-air missile downed a Malaysian Air airliner carrying some 300 passengers.

Whereas Ukrainian forces on Crimea did not resist the Russian invasion (in part at the urging of the West), Kyiv resisted the appearance of little green men in Donbas. Before long, the Ukrainians found themselves fighting Russian troops as well as “separatist” forces. That conflict is now about to enter its sixth year.

Finding a settlement in Donbas has taken higher priority over resolving the status of Crimea—understandable given that some 13,000 have died and two million been displaced in the fighting in eastern Ukraine. Moscow seems to see the simmering conflict as a useful means to pressure and distract Kyiv, both to make instituting domestic reform more difficult and to hinder the deepening of ties between Ukraine and Europe.

Resolving the Donbas conflict will not prove easy. For example, the Kremlin may not be prepared to settle until it has some idea of where Ukraine fits in the broader European order, that is, its relationship with the European Union and NATO. But Russia has expressed no interest in annexing Donbas. While the seizure of Crimea proved very popular with the broader Russia public, the quagmire in Donbas has not. The most biting Western economic sanctions would come off of Russia if it left Donbas. At some point, the Kremlin may calculate that the costs outweigh the benefits and consent to a settlement that would allow restoration of Ukrainian sovereignty there.

Moscow will not, on the other hand, willingly give up Crimea. Russians assert a historical claim to the peninsula; Catherine the Great annexed the peninsula in 1783 following a war between Russia and the Ottoman Empire. (That said, Crimea was transferred from the Russian Soviet Federative Socialist Republic to the Ukrainian Soviet Socialist Republic in 1954, and, as noted above, the republics that emerged from the wreckage of the Soviet Union in 1991 agreed to accept the borders as then drawn.)

Retaining Crimea is especially important to Putin, who can offer the Russian people no real prospect of anything other than a stagnant economy and thus plays the nationalism and Russia-as-a-great-power cards. He gained a significant boost in public popularity (much of which has now dissipated) from the rapid and relatively bloodless takeover of the peninsula. Moreover, it offers a vehicle for Russia to maintain a festering border dispute with Ukraine, which the Kremlin may see as discouraging NATO members from getting too close to Ukraine.

Kyiv at present lacks the political, economic, and military leverage to force a return. Perhaps the most plausible route would require that Ukraine get its economic act together, dramatically rein in corruption, draw in large amounts of foreign investment, and realize its full economic potential, and then let the people in Crimea—who have seen no dramatic economic boom after becoming part of Russia—conclude that their economic lot would be better off back as a part of Ukraine.

For the West, Russia’s seizure and annexation of Crimea pose a fundamental challenge to the European order and the norms established by the 1975 Helsinki Final Act. The United States and Europe should continue their policy of non-recognition of Crimea’s illegal incorporation. They should also maintain Crimea-related sanctions on Russia, if for no other reason than to signal that such land grabs have no place in 21st-century Europe.