WASHINGTON — For years, U.S. officials have tiptoed around the question of how much military support to provide to Ukraine, for fear of provoking Russia.
Now, in what would be a major turnaround, senior Biden administration officials are warning that the United States could throw its weight behind a Ukrainian insurgency should President Vladimir V. Putin of Russia invade Ukraine.
How the United States, which just exited two decades of war in Afghanistan, might pivot to funding and supporting an insurgency from fighting one is still being worked out. But even a conversation about how far the United States would go to subvert Russian aims in the event of an invasion has revived the specter of a new Cold War and suddenly made real the prospect of the beginnings of a so-called great power conflict.
In Afghanistan, the United States showed itself to be dismal at fighting insurgencies. But when it comes to funding them, military experts say it is a different ballgame.
President Biden has not determined how the United States might arm insurgents in Ukraine who would conduct what would amount to a guerrilla war against Russian military occupation. Nor is it clear what Russia’s next move might be, or whether Mr. Putin intends to launch a cyberattack. On Friday, hackers brought down several Ukrainian government websites, after days of talks between Russia and the West about the crisis.
But Biden administration officials have begun signaling to Russia, which has massed about 100,000 troops at its borders with Ukraine, that even if it managed to swiftly capture territory, Mr. Putin would eventually find the costs of an invasion prohibitively expensive in terms of military losses.
“If Putin invades Ukraine with a major military force, U.S. and NATO military assistance — intelligence, cyber, anti-armor and anti-air weapons, offensive naval missiles — would ratchet up significantly,” said James Stavridis, a retired four-star Navy admiral who was the supreme allied commander at NATO. “And if it turned into a Ukrainian insurgency, Putin should realize that after fighting insurgencies ourselves for two decades, we know how to arm, train and energize them.”
He pointed to American support for the mujahedeen in Afghanistan against the Soviet invasion there in the late 1970s and 1980s, before the rise of the Taliban. “The level of military support” for a Ukrainian insurgence, Admiral Stavridis said, “would make our efforts in Afghanistan against the Soviet Union look puny by comparison.”
Both Defense Secretary Lloyd J. Austin III and Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, have warned their Russian counterparts in recent telephone calls that any swift Russian victory in Ukraine would probably be followed by a bloody insurgency similar to the one that drove the Soviet Union from Afghanistan. In discussions with allies, senior Biden officials have also made clear that the C.I.A. (covertly) and the Pentagon (overtly) would both seek to help any Ukrainian insurgency.
Administration officials interviewed this week said that plans to help Ukrainian insurgents could include providing training in nearby countries that are part of NATO’s eastern flank: Poland, Romania and Slovakia, which could enable insurgents to slip in and out of Ukraine. Beyond logistical support and weapons, the United States and NATO allies could also provide medical equipment, services and even sanctuary during Russian offensives. The United States would almost certainly supply weapons, the officials said.
Since Russia invaded Crimea in 2014, successive U.S. administrations have taken pains to limit military support to Ukraine largely to defensive weaponry. The United States has provided about $2.5 billion in military aid to Kyiv, including anti-tank missiles and radar that enables the Ukrainian military to better spot sources of artillery fire. The assistance has also included patrol boats and communications equipment.
The United States is also moving toward providing Ukraine with battlefield intelligence that could help the country more quickly respond to an invasion, senior administration officials said.
But all of that aid has been calibrated not to provoke Mr. Putin, officials said. If Russian troops crossed the border, the officials said, the United States could offer offensive weaponry and training.
“Given the right equipment and tactics, Ukraine can dramatically reduce the chances of a successful invasion,” a former Ukrainian defense minister, Andriy P. Zagorodnyuk, wrote in an op-ed for the Atlantic Council on Sunday that reads like an instructional manual for how the United States can support an insurgency. “By combining serving military units with combat veterans, reservists, territorial defense units and large numbers of volunteers, Ukraine can create tens of thousands of small and highly mobile groups capable of attacking Russian forces. This will make it virtually impossible for the Kremlin to establish any kind of administration over occupied areas or secure its lines of supply.”
But it is difficult to know whether Ukrainians would be willing to start an insurgency campaign that could drag on for years or even decades. Some Ukraine experts point to Crimea, where there has been little armed resistance since Russia invaded. And Mr. Putin could limit his siege to the eastern parts of Ukraine, which lean more pro-Russian than the west.
Understand the Escalating Tensions Over Ukraine
A brewing conflict. Antagonism between Ukraine and Russia has been simmering since 2014, when the Russian military crossed into Ukrainian territory, annexing Crimea and whipping up a rebellion in the east. A tenuous cease-fire was reached in 2015, but peace has been elusive.
A spike in hostilities. Russia has recently been building up forces near its border with Ukraine, and the Kremlin’s rhetoric toward its neighbor has hardened. Concern grew in late October, when Ukraine used an armed drone to attack a howitzer operated by Russian-backed separatists.
Ominous warnings. Russia called the strike a destabilizing act that violated the cease-fire agreement, raising fears of a new intervention in Ukraine that could draw the United States and Europe into a new phase of the conflict.
The Kremlin’s position. President Vladimir V. Putin of Russia, who has increasingly portrayed NATO’s eastward expansion as an existential threat to his country, said that Moscow’s military buildup was a response to Ukraine’s deepening partnership with the alliance.
A measured approach. President Biden has said he is seeking a stable relationship with Russia, though tension has been rising. So far, his administration is focusing on maintaining a dialogue with Moscow, while seeking to develop deterrence measures in concert with European countries.
A Western military adviser to the Ukrainians said that details of a specific resistance there remained a closely held secret. But already, particularly in the west, Ukrainians are joining territorial defense forces that train in guerrilla tactics.
The Biden administration and its NATO allies want to capitalize on any distaste the Russian body politic might have for troop casualties, U.S. and European officials said in interviews. The officials spoke on the condition of anonymity because they were not authorized to publicly discuss planning.
Funding and supporting an insurgency is difficult, defense planners and military experts said. It requires developing a long-game mind-set and keeping up morale among fighters in the face of a state-sanctioned enemy that is backed by heavy weaponry.
Russian retaliation against Ukrainian insurgents is likely to be “swift, direct and very brutal,” said Seth G. Jones, the director of the international security program at the Center for Strategic and International Studies. “It’s likely to get bloody.”
Mr. Jones also suggested that Russia might start to build some type of wall along Ukraine’s borders with NATO countries, putting a new Iron Curtain in place.
But Evelyn N. Farkas, who was the deputy assistant secretary of defense for Russia, Ukraine and Eurasia in the Obama administration, when Russia invaded Crimea, recalled a debate at the time about how tough the United States could be with Russia without provoking further action. Eight years later, she noted that Mr. Putin is still threatening the country’s sovereignty.
In an interview, she said that this time around, “I think the gloves should come off.”
Eric Schmitt contributed reporting.