Warnings from the West are becoming ever clearer, with many seeing a Russian invasion of Ukraine as a growing possibility.

Russia has amassed 100,000 troops on its border with Ukraine and NATO is sending ships and fighter jets into Eastern Europe — in direct contravention of the Kremlin's demands, which wants those forces removed.

For Europe, there presents a serious conundrum: What to do if Russian troops cross the border and invade Ukraine?

European Union foreign ministers agreed at a meeting on Monday that "any further military aggression by Russia against Ukraine will have massive consequences and severe costs," according to a joint statement.

We have heard this many times by now. But what consequences, exactly, are we talking about?

Sanctions and actions

At a summit in mid-December, EU heads of state and government agreed in principle on punitive measures against Russia, but disagreed about what those measures could be exactly.

Germany, Austria and Hungary are more closely linked to the Russian economy than, say, Portugal or the Netherlands; and, because the EU requires unanimity to pass any political actions, the only thing that the European Commission has at the moment is a list of possible sanctions, tucked away in a drawer in Brussels.

EU diplomats who have been busy preparing punitive sanctions have said these sanctions would have to be implemented within 48 hours of an invasion by Russia. These would have to be coordinated with the United States and the United Kingdom, which has also announced unspecified plans for sanctions.

It remains unclear whether those possible sanctions include plans to sever Russia from the global electronic payments system, SWIFT or end the Russian-German gas pipeline Nord Stream 2.

"Putting tough sanctions on Russia can also have consequences for the EU because the economies are linked," said Amanda Paul, a security expert at the European Policy Centre, a think tank that fosters European integration.

"There could be costs to pay that some member states do not want to pay," she added. Paul said officials would need to consider the possibility that Ukrainian refugees could be forced into EU territory and that destabilization of the country could destabilize the entire Black Sea region.

Gustav Gressel, a senior policy fellow at the European Council on Foreign Relations, said that only really severe sanctions will change Putin's position.

"He has priced in and extensively prepared Russia for economic sanctions," Gressel told DW. "Only harsh sanctions against the energy sector would really hurt Russia — other sanctions are rather a disturbance than an obstacle, and Moscow is confident it can work around them."

Specify 'invasion'

Currently, one of the main debates in Brussels centers around the need to clarify what exactly triggers EU sanctions.

Would that include a military invasion involving Russian troops crossing Ukraine's border? Or the infiltration of "little green men," a reference to soldiers lacking national insignias on their green uniforms, as was the case during the 2014 invasion of Crimea? 

Or what about an increase in fighting in eastern Ukraine, where separatists have been able to hold territory since 2014 thanks to Russian backing? Or would a cyberattack on Ukraine’s critical infrastructure fit the bill? Such matters remain unclear.

Either way, experts agree the response by the US and European partners needs to be unified.

Ian Lesser, vice president of The German Marshall Fund of the United States, told DW: "Without US-European cohesion, Moscow will have — or at least will feel it has — a blank check."

For Russia's strategy, "Driving a wedge between trans-Atlantic partners is likely at least as important as Ukraine itself."

NATO holds to 'open door' policy

Russia is demanding that NATO withdraw its troops from Eastern Europe, in addition to assurances that Ukraine and Georgia will never become NATO members, before it withdraws its troops from the border.

NATO has repeated its position that every country's security decisions are sovereign choice and that its "open door" policy will never be changed.

Lesser believes different positions are being expressed behind closed doors, however. "In truth, there is very little enthusiasm in the EU or NATO for Ukrainian membership," he said. "Some may see commitments to Ukraine as a security liability."

When it comes to protecting member states from aggression by Russia, the NATO alliance, to which most EU members belong, is however clear on one thing: Though Ukraine has aspirations of joining NATO, it currently isn't a member and therefore doesn't fall under its protective umbrella.

"We will always respond in a determined way to any deterioration of our security environment, including through strengthening our collective defense posture as necessary," the alliance's decision-making body, the North Atlantic Council, said in December.

The alliance said it is in a position to activate its Very High Readiness Joint Task Force, which comprises some 5,000 soldiers, on short notice. Within 30 days, this first "spearhead" could expand to up to 40,000 soldiers in what the alliance calls its NATO Response Force.

"We will provide Ukraine with further military assistance, equipment and defensive weapons in the coming weeks," said US Assistant Secretary of State Karen Donfried last month. "If Russia continues with an invasion," she added, "we will provide more."

The response force's mission would be to defend NATO territory. Germany currently has 16,000 troops assigned to the response force.

Regional troop presence could increase

Meanwhile, US President Joe Biden is mulling whether he should send several thousand troops to Eastern Europe.

Secretary of State Antony Blinken told CBS News on Sunday that the US is "very much focused on building up defense, building up deterrence."

US officials have said the plans being considered would not include deployments to Ukraine itself, which would be seen in Moscow as a major provocation.

Instead, the additions would likely go to the Baltic NATO members of Latvia, Lithuania and Estonia — among the countries most worried about Russia's actions. The size of the deployment could be somewhere between 1,000 and 5,000 US military personnel.

In December, Germany's new Defense Minister Christine Lambrecht chose to first officially visit Lithuania, where NATO forces have been sending in reinforcements over recent years to counter a potential threat from Russia.

"The situation in Ukraine is very serious, and I can understand the concerns of our Baltic allies and understand if one feels threatened," Lambrecht said.

Edited by: Sonya Diehn