Underfunded, under-equipped and with its senior leadership often seen as close to Moscow, Ukraine's military has been something of a no-show in its confrontation with Moscow.
But that, experts say, has probably been largely deliberate.
Ukrainian forces have been involved in some standoffs with Russian forces particularly around bases in Crimea, some of which have long been shared between Kiev and Russian forces attached to the Black Sea Fleet.
On Sunday, Ukraine's newly appointed navy commander publicly defected to Crimea's regional administration, long Russian dominated and now effectively under Moscow's control.
Ukraine's senior military leadership served much of their careers in the Soviet Army with their Russian colleagues before the breakup of the Soviet Union in 1991. Neither they nor their new broadly pro-Western leadership in Kiev are keen to provoke a confrontation they know they cannot win.
"I think there are strong directives from Kiev in place to avoid confrontation," says Dmitry Gorenburg, regional analyst for the U.S. government-funded Center For Naval Analyses, part of the larger not-for-profit CNA Corporation.
"This actually fits well with commanders' inclination because of their familiarity with the Russians — especially in Crimea, where they are essentially co-located."
A single incident could still cause further escalation, he said.
Overwhelming Russian Superiority
On Monday night, Ukraine's ambassador to the United Nations said Moscow now had moved some 16,000 troops onto the Crimean peninsula. Having mobilized up to 150,000 personnel last week for what it said at the time were exercises, President Vladimir Putin ordered them back Tuesday morning, though later said Russia could use them if the situation in Ukraine put citizens in danger.
London's International Institute for Strategic Studies, or IISS, think tank estimates Russia has some 845,000 military personnel alongside a 2 million-strong reserve of those with recent military training.
Since the 2008 Georgia war, it has increased military spending by more than 30 percent to some $68 billion a year.
Based in Sevastopol, its Black Sea Fleet contains three submarines, seven destroyers, five frigates, 20 patrol boats and corvettes, 15 minesweepers and 11 amphibious craft.
Other naval forces have also operated from the port, which has been central to supporting recent naval operations in the Mediterranean and against Somali pirates in the Indian Ocean.
IISS estimates Russia has almost 1,400 combat aircraft of various types, though some are outdated by Western standards.
IISS estimates Ukraine spends some $2.4 billion a year on defense but has struggled to keep its armed forces up to date.
Its most elite units have experience of fighting alongside NATO in Afghanistan and elsewhere on international peacekeeping operations. Most of its personnel, however, have too little experience of modern post-Cold War equipment.
It has an estimated 129,950 service personnel with 64,750 in the Army, 13,950 in the Navy, 45,250 in the Air Force and another 6,000 in separate airborne units. Before the recent troubles, its paramilitary forces registered some 84,900 though that number is expected to have fallen with demobilization of elite riot units accused of killing unarmed protesters.
Moscow mobilized some 150,000 troops last week before moving into Ukraine, many of them recently re-equipped units.
Ill-Maintained Navy, Air Force
Largely housed in bases alongside or next to the much more powerful Russian Black Sea Fleet, the Navy has little or no ability to deploy to sea unless allowed by Moscow.
Poor maintenance and a shortage of spares have kept its single Russian-built Foxtrot-class diesel submarine out of the water. It has a single frigate, recently deployed to NATO counter-piracy operations off Somalia, another 10 patrol boats and Corvettes, five minesweepers, five landing ships and aircraft and several dozen other support vessels.
Many of those are believed out of service. Similar problems afflict its Cold War-era jet fleet that seemed unable to stop Russian jets, according to Kiev, violating its airspace.
On paper, Kiev has 121 combat aircraft including MiG-29 and Su-27 jet fighters and older Su-24 and Su-25 attack aircraft. In reality, many are grounded.
IISS estimates its pilots get only 40 hours flying time or so a year compared to 60 to 100 for their Russian counterparts.
President Vladimir Putin ordered tens of thousands of Russian troops participating in military exercises near Ukraine's border to return to their bases on Tuesday.
The massive military exercise in western Russia involving 150,000 troops, hundreds of tanks and dozens of aircraft was supposed to wrap up anyway, so it was not clear if Putin's move was an attempt to heed the West's call to de-escalate the crisis that has put Ukraine's future on the line.
On Tuesday, Russian troops who had taken control of the Belbek air base in the Crimea region fired warning shots into the air as about 300 Ukrainian soldiers, who previously manned the airfield, demanded their jobs back.
About a dozen Russian soldiers at the base warned the Ukrainians, who were marching unarmed, not to approach. They fired several warning shots into the air and said they would shoot the Ukrainians if they continued to march toward them.
The shots reflected tensions running high in the Black Sea peninsula since Russian troops — estimated by Ukrainian authorities to be 16,000 strong — tightened their grip on the Crimean peninsula over the weekend.
There was no fighting elsewhere in Crimea early on Tuesday. A supposed Russian ultimatum for two Ukrainian warships to surrender or be seized passed without action from either side, as the two ships remained anchored in the Crimean port of Sevastopol. Russian Defense Ministry spokesman Vladimir Anikin said late Monday that no ultimatum had been issued.
U.S. President Barack Obama has said that Russia is "on the wrong side of history" in Ukraine and its actions violate international law. Obama said the U.S. was considering economic and diplomatic options that will isolate Russia, and called on Congress to work on an aid package for Ukraine.
(The Associated Press)