It seems like one reason why Russia’s actions in Crimea appear so jarring and brazen is that it’s a form of warfare that was once common but rarely take place anymore. Russia may not formally annex Crimea – it seems more likely that the territory will declare independence under heavy Russian influence – but it has essentially invaded another country to lob off a piece of territory that was, despite longstanding nationalist sentiment, an undisputed part of Ukraine.
Historically speaking, conflicts in which one country sends troops into the territory to take over a disputed region are pretty common. But today, interstate war is relatively rare, and interstate wars over control of territory even rarer. For the most part, conflicts today usually take place between armed groups within states, and when one country does send troops into another – the U.S. in Iraq and Afghanistan, for instance – it’s generally under the assumption that sooner or later they will pull out, leaving borders as they are.
In fact, the intense emotions aroused in Japan, China, and South Korea over handfuls of small islands and reefs only highlights the degree to which countries rarely resort to armed conflict over large inhabited areas anymore.
Russia’s 2008 invasion of Georgia was something of an exception, although – to a greater extent than Crimea – the breakaway regions of Abkhazia and South Ossetia were already outside of Georgian control. Thailand and Cambodia have fought some inconclusive border disputes. Sudan and South Sudan have also continually fought over their still disputed border. The Israeli-Palestinian conflict is, of course, a fight about land, but land hasn't actually changed hands in quite some time.
Other than that, the last major wars of this type were in the early 1990s – between the new former Yugoslavian states, some of the former Soviet Republics, and of course Iraq’s short-lived annexation of Kuwait.
Given the amount of blood spilled over tracts of land in just the twentieth century, we should certainly hope Crimea is an anomaly rather than a sign that countries are returning to the old way of doing business.