Russia has invaded the Crimean region of the sovereign nation of Ukraine with a military force, many of whom are not wearing appropriate military insignia, that numbers more than 15,000.
Ukraine is divided between a Ukrainian-speaking majority and a significant minority of Russian speakers. There are deep historical ties between Ukraine and Russia going back centuries, and the linkages are particularly strong in the small peninsula of Crimea, which is home to the Russian Black Sea Fleet — one-fourth of the entire Russian Navy. The ships and sailors reside in the area under a long-term lease between Russia and Crimea.
See also: 5 Big Questions About the Future of Ukraine
The justification for the invasion, as Russian President Vladimir Putin laid out on Tuesday, is to protect Russian citizens in Crimea, ensure the safety of the Black Sea Fleet, protect the Russian-speaking minority of the country and stabilize the political situation.

Ukrainian officers wait for the results of the talks between their commanding officers and Russian troops at the Belbek air base, outside Sevastopol, Ukraine, on Tuesday, March 4, 2014.
Russia also wants a return to the previously negotiated settlement between the current pro-European Union regime in Kiev and the pro-Russian regime of elected President Yanukovich, whose regime was driven from power after firing with snipers on crowds of protestors in Kiev several weeks ago. The new interim, pro-EU regime intends to hold elections in May to legitimize and stabilize the situation in a matter of months (with help from the EU).
The West in general, as well as the United States in particular, has condemned the actions of Russia in invading a sovereign nation without reference to the United Nations. This is a clear violation of international law in the view of most of the world outside of Russia. Even the Chinese, normally staunch allies of Russia in such situations, seem to be distancing themselves from the invasion.
Next steps for the West will include strong political condemnation, perhaps expelling Russia from the G8 group, economic sanctions — probably designed at weakening the Ruble — and, potentially, personal penalties of senior Russian public and political figures. NATO will probably enhance intelligence collection, conduct information sharing and the advising of Ukrainian forces, and potentially provide logistic and other forms of support to them.

Located in southern Ukraine, the Crimea region is largely made of of Russian-language speakers.
Ukraine has been a strong partner to NATO in Afghanistan, the Balkans, Libya, and on piracy missions off east Africa. NATO may decide to work on contingency plans in case of further aggression on the part of Moscow, especially guarding against the possibility of such a move toward a NATO ally.
During my time as Supreme Allied Commander at NATO from 2009 to 2013, I had the chance to see Ukrainian troops operating alongside NATO troops all over the world. They are tough, dedicated and reasonably capable. While their equipment and general level of training will not stand up to Russian mainstream forces, they are spirited and can be very determined.

Image: Mikhail Metzel/Associated Press

It seems unlikely that Russia will withdraw its forces without a very strong reaction from the West. Many observers believe that Putin's overall strategic objective is to at a minimum set up a sort of "Republic of Crimea" that would be dominated by Russia, much like the enclaves carved out of Georgia in 2008, Abkhasia and Osseita.
The situation is very volatile and dangerous, and the potential for combat between Russian forces and the Ukrainian military is deeply worrisome.
Hopefully the United Nations will be able to broker an agreement that sees Russia stand down and withdraw its troops, with negotiations to follow to protect Russian citizens and property in Ukraine. Until then, the West will probably ratchet up the pressure on Russia as the stand-off continues in the historic Crimean peninsula.
Admiral James Stavridis (Ret.) is the dean of The Fletcher School of Law and Diplomacy at Tufts University, and served as Supreme Allied Commander at NATO from 2009-2013.