* European defence cuts, U.S. withdraws questioned

* NATO leaders to discuss cuts at September summit - UK

* States will find it hard to fund more defence spending

* Washington may stop force withdrawals from Europe

* U.S. would struggle with simultaneous Russia, China crises (Adds comments from NATO secretary-general in paragraphs 4-5)

By Peter Apps and Adrian Croft

LONDON/BRUSSELS, March 19 (Reuters) - For defence planners in Washington, London and Brussels, the sight of Russian forces pouring into their second neighbour in six years will overturn two decades of strategic assumptions.

The result of Russia's seizure of Crimea from Ukraine, following its 2008 war with Georgia, could be a modest reversal of years of European defence cuts and a bigger U.S. military presence in the NATO members of central and eastern Europe.

Since the end of the Cold War, the Western alliance has shifted its attention to Afghanistan, Kosovo and counter-piracy operations off the coast of Somalia, as well as Libya during its 2011 civil war. But by the time NATO government leaders meet in September in Wales, some people believe their focus will have returned to deterring Moscow.

In Washington on Wednesday for meetings with senior officials, NATO Secretary-General Anders Fogh Rasmussen accused Moscow of acting to "rip up the international rulebook, trying to redraw the map of Europe and creating... the most serious security crisis since the end of the Cold War".

"It goes to the heart of what NATO is about," he told a forum at Georgetown University.

While a major war in Europe remains extremely unlikely, it is no longer unthinkable, say officials and analysts. But finding the money for more military resources will be tough.

In the past fortnight, Washington and NATO have tried to reassure members of the alliance that were once part of the Soviet bloc, such as Poland and the Baltic states. Sending a message that NATO stands with them in any confrontation with Moscow, the United States deployed F-15 fighters to Lithuania and alliance early warning aircraft have increased patrols. While some Western governments regarded Russia's war with Georgia as a one-off, they see its annexation of Crimea as a sign of things to come.

Since 2008 European Union states have cut their military budgets by about 15 percent, according to a report last year from the Centre for European Reform, whereas Russia has increased its by about 30 percent.

While Moscow lacks its Cold War-era strength to overrun much of Europe, President Vladimir Putin seems increasingly confident in intervening in his neighbourhood.

"This requires a complete reappraisal of how we approach Russia," says Fiona Hill, U.S. national intelligence officer for Russia from 2006 to 2009, who now heads the Europe program at the Brookings Institution in Washington. "Putin has made it very clear he intends to reassert Russia's sphere of influence ... We don't have a strategy to deal with that."

Putin has said Sunday's Crimean referendum, which Kiev and the West have refused to recognise, showed the overwhelming will of the people to be reunited with Russia. Moscow has also said that worries it might now move on Russian-speaking areas in eastern Ukraine are unjustified.

Western officials and analysts say there is little NATO can do to stop Moscow in former Soviet states that are outside the alliance, such as Ukraine and Georgia, although some say NATO membership for them might come back on the agenda.

The potential flashpoint, however, is the Baltic states. These former Soviet republics are now in NATO and therefore protected by Article 5 of its treaty, which requires all members to help an ally under attack. Like Ukraine, they are home to significant Russian minorities.

"Western nations have minimised the prospect of having to reinforce our eastern allies," said one senior Western official on condition of anonymity. "At a stroke, all that has changed."

ERA OF WAR IN EUROPE "NOT OVER"

Another NATO diplomat put it more strongly.

"I think people do understand continental wars in Europe are not over," he said on condition of anonymity. "We will not be credible if we simply continue as if nothing has happened."

U.S. forces in Europe have shrunk dramatically since the Cold War. Their numbers are about 80,000, including 14,000 civilian staff, according to the U.S. military's European Command, down from just over 300,000 in the last decades of the Soviet Union.

Speaking on condition of anonymity, U.S. officials played down suggestions of a major shift in America's European presence following events in Crimea. Other officials and analysts, however, say a change in strategic mood is already underway.

Reversing defence cuts is likely to be a major topic of the NATO summit, British Foreign Secretary William Hague told Sky News earlier this month. Some analysts are sceptical.

"Frankly there is no appetite for increasing defence spending," said Judy Dempsey, senior associate at Carnegie Europe, noting that despite Hague's comments there were no signs of Britain reversing its defence cuts.

The most likely scenario, she said, was for countries to pool more resources and aim for greater efficiency.

Some countries such as Germany might put off planned further cuts, while U.S. force withdrawals could also cease.

Some analysts had expected the Pentagon to pull a squadron of F-15 fighters out of their base in Lakenheath, England, as it prioritises the Pacific. That now looks less likely - 10 of the squadron's aircraft are in the Baltic states.

Ultimately, some Western officials privately say Washington and perhaps others may end up with a permanent presence in Eastern Europe. That would overturn an unwritten agreement with Moscow not to base U.S. forces in former East bloc states.

NUCLEAR WORRIES

In the shorter term, temporary deployments and training missions look likely to increase dramatically. "Russia is not posing a mass army threat but rather the ability to selectively use its local military advantages decisively, backed up by the threat to escalate," says Elbridge Colby, a former Pentagon official who is now at the Center for a New American Security. "The corresponding NATO response needs to be to present a deterrent to Russia's ability to pull this off against NATO members."

Last year, NATO conducted one of its largest recent exercises, "Steadfast Jazz" in Poland and Latvia, deploying its rapid response brigade in a show of force that some officials said was aimed at reassuring local states. That followed a major Russian exercise code-named "Zapad-13" in which 10,000 Russian troops fought "Baltic terrorists" in Belarus.

Nuclear weapons are also back on the agenda.

With the rhetoric rising during the Crimean crisis, a senior Kremlin-backed broadcaster made an explicit nuclear threat this week, saying Russia remained "capable of turning the United States into radioactive ash".

That could have an effect on those countries - Germany, the Netherlands, Belgium, Italy and Turkey - with U.S. nuclear weapons still on their soil. This arrangement had seemed anachronistic and there had been talk of ending it. Experts say NATO faces another awkward reality with conventional weapons. In the event of simultaneous crises with Russia and China - perhaps over the Baltic states and disputed South China Sea islands respectively - Washington would probably struggle to reinforce both regions.

"The week before Russia went into Crimea, we published our defence budget and Quadrennial Defence Review," said Nikolas Gvosdev, professor of national security studies at the U.S. Naval War College. "That accepted a risk of war with China. But not Russia." (Additional reporting by Phil Stewart in Washington; Editing by David Stamp and Lisa Shumaker)

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