Redrawing national borders may feel like a historical relic that belongs to an earlier century, yet Crimea's crisis shows there are still places that don't fit neatly on the map and may not for years to come.
Just last month, Crimea was part of Ukraine. On Sunday, Crimeans vote on whether they want to become part of Russia. Nevermind that the rest of the world rejects the validity of the ballot; no country appears willing or able to prevent Crimea from leaving Ukraine and joining Russia.
This strikes many as a dangerous development on a continent that's had its borders erased and reconstituted countless times.
"This is the first time since 1945 when a great power has changed, or is about to change, Europe's borders by force," Josef Joffe, editor of the German newspaper Die Zeit, told NPR's Morning Edition.
"It is a very serious test of wills which will decide whether Europe lives by the rules it has written the last 70 years or whether we're going back into the kind of 19th and 18th century," he added.
Borders do occasionally shift in the modern world, but the circumstances vary and the rules are fuzzy. In general, the international community rejects the use of brute force and accepts revised boundaries that are mutually agreed upon.
But often the cases fall somewhere in between and can drag on unresolved for decades. Here's a short list:
If a small, plucky territory is trying to cut itself loose from a larger power that's viewed as oppressive, then the international community may offer its blessing. But there's a lot of subjectivity involved.
The 1991 breakup of the Soviet Union is the greatest single source of current borders disputes, with at least a half-dozen territorial conflicts that are still unresolved.
Russia has expressed great empathy for ethnic Russians in Crimea who want to break away from Ukraine. On the other hand, Russia has fought two brutal wars in Chechnya to prevent that small territory from separating itself from Russia.
Another recent example is Kovoso's 2008 declaration of independence from Serbia. NATO intervened a decade earlier in support of the Kosovars following widespread abuses carried out under Serbian leader Slobodan Milosevic.
The U.S. and many other countries supported Kosovo's independence, though international backing was far from universal. For instance, Russia has opposed Kosovo's independence and has the power to veto an application for United Nations membership.
The model here is Czechoslovakia's 1989 Velvet Revolution that led to the peaceful creation of the Czech Republic and Slovakia.
"When Czechoslovakia split up, the national governments agreed. This is the kind of breakup you always want, but you rarely get," says Stephen Larrabee of the Rand Corp.
In a similar vein, Scotland will hold a referendum in September on whether it wants to opt for independence after being part of the United Kingdom for more than three centuries. The polls suggest most Scots favor continued union.
However, this approach does not guarantee a happy ending. With the help of the United Nations and the international community, South Sudan gained independence from Sudan in 2011 after decades of war. The hope was that both sides could peacefully go their own way, but the two countries have continued to feud. South Sudan appeared on the brink of civil war before a cease-fire was signed in January, but the country remains unstable.
Disputes Spanning Generations
Israel has never had fixed borders since its independence in 1948, and the Palestinians have never had a state at all.
India and Pakistan have been at odds over Kashmir since those two countries gained independence in 1947, fighting wars and posting soldiers on a Himalayan glacier along the Line of Control that separates the two sides.
Perhaps the question here is what role the international community should play in these disputes.
The U.S. and many others have sought to broker peace between the Israelis and Palestinians, to no avail. The world has mostly left the Indians and the Pakistanis to their own devices, and the conflict also remains stubbornly unresolved more than six decades later.
Greg Myre is the international editor at NPR.org. You can follow him on Twitter @gregmyre1