The worst-case scenario in the Russia-Ukraine crisis would be a war between the two states. How do their respective forces compare?
What is the balance of forces between the two sides?
Russia has about four times as many soldiers as Ukraine, twice as many tanks, and more than six times as many combat aircraft. The huge imbalance in forces reflects the defense budgets of the two countries. Russia spends about $78 billion on its armed forces annually, Ukraine $1.6 billion.
However, only a part of Russia's forces is available to deploy against Ukraine. Moscow cannot afford to remove forces from its North Caucasus region, from its border with China, or from the Pacific.
Mark Galeotti, a Moscow-based regional expert with the Center for Global Affairs at New York University, estimates Russia might be able to muster a force twice the size of Ukraine's for any war. That leaves open the possibility that the difference in strength might not be so overwhelming as to guarantee victory.
What about the two sides operational capabilities?
Since the 2008 Georgia war, Moscow has stepped up its military spending by 30 percent, reflecting the importance it gives to modernizing its forces and using its military as an arm of its foreign policy.
By contrast, Kyiv has a history of delivering less money to its armed forces than budgeted. Its most elite units have deployed alongside NATO in international peacekeeping operations but the rest are underfunded and underequipped. Most of Ukraine's equipment is what it inherited when it declared independence from the Soviet Union more than two decades ago.
Christopher Langton of Independent Conflict Research and Analysis in Britain says the Ukrainian military has also been shaken by reforms intended to make it a more professional force but which remain incomplete.
"They have undergone major upheavals in recent years, major reforms, [but with] very inadequate financing of equipment programs and personnel reform programs," Langton says. "And they tried to end conscription but this was never achieved, so there is still an element of conscription in the Ukrainian armed forces and this, of course, weakens the deployability of the army."
Over the past five years the size of Ukraine's military force has shrunk from 245,000 to 184,000. But it still includes about 60 percent conscripts.
Would Ukraine's army split in the face of an invasion?
The question is raised by the fact Ukraine's population includes both native Ukrainian speakers and native Russian speakers and many in the latter group, particularly in the east of the country, have close ties to Russia. That could create a regional pull on the loyalty of Ukraine's soldiers and officers.
There are less obvious factors to consider, too. Ukraine's senior military leaders served much of their careers in the Soviet Army with their Russian counterparts. The possibility of defections was highlighted when Ukraine's newly appointed navy chief declared allegiance to the pro-Russian Crimea region on March 2 as Russian troops took control of the peninsula.
Still, the Ukrainian Army has two glues to keep it together. One is the heightened sense of national identity an invasion by any foreign force helps create. The other is the strong sense of identity of the Ukrainian army itself.
"The Ukrainian military has evolved really quite a long way from its Soviet roots," Galeotti notes. "It has got quite a strong esprit de corps, quite a strong culture of service to the state."
What form might combat take?
The question is what strategy Moscow might choose. It could attempt a full-scale military invasion of Ukraine, along the lines of the Warsaw Pact invasion of Czechoslovakia in 1968. Or it could try to replicate its successful tactics in Crimea of encouraging so-called "self-defense forces" to take over a region while mixing its own unidentified troops among them.
Langton says that a full-scale invasion would almost certainly precipitate a conventional state-to-state war that could prove too costly to Moscow to pursue.
"Although on paper these two forces are imbalanced in terms of capabilities, if it came to a full-scale Russian military operation in Ukraine, this would be on the territory of Ukrainians and they would fight," the British analyst explains. "There would probably be a fairly bloody struggle which would not be acceptable to the Russians, and this would be followed by, maybe, a period of partisan or guerilla-type warfare which, of course, the defending forces would be able to conduct on their own territory."
The cost of such a direct showdown might lead Moscow to prefer the stealthier strategy of wresting control of individual regions, particularly in heavily Russian-speaking eastern Ukraine, under the guise of interceding in supposedly local conflicts. That strategy caught Kyiv flat-footed in Crimea and places Ukraine in the weak position of having to retake seized land.
In either case, Ukraine is not immediately well prepared to cope with an attack by Russia. Thanks to the legacy of the Soviet Union, its infrastructure of military bases remains configured to support a ground war against a western invader, not an eastern one.
With additional reporting by Ron Synovitz