WASHINGTON — A Russian missile strike on Sunday in western Ukraine near the Polish border, which has become a transit point for weapons being sent to Ukrainian forces, raised new concerns about whether Russia might attack NATO territory to stop or destroy the shipments.
Such an attack could dramatically expand the conflict. The arms — which include machine guns, tactical drones and antitank missiles — have passed through Poland and Romania, both NATO members, and an attack on either country would activate the alliance’s collective-defense provision, known as Article 5. Speaking in Warsaw on Saturday, President Biden said the United States had a “sacred obligation” to honor Article 5.
The prospect of direct conflict with the nuclear-armed NATO alliance provides President Vladimir V. Putin of Russia with a powerful disincentive.
But modern history, including the Soviet Union’s experience in Afghanistan in the 1980s, shows that even in the absence of a possible nuclear war, invading powers have been surprisingly hesitant to target countries that provide adversaries with lethal support.
“There just historically has always been a reluctance to broaden these conflicts to surrounding nations,” said Bruce Hoffman, a counterinsurgency expert and a professor at Georgetown University’s Walsh School of Foreign Service.
Russia’s foreign minister, Sergey V. Lavrov, seemed to threaten such an attack, recently telling the state-run news outlet RT that “any cargo moving into Ukrainian territory which we would believe is carrying weapons would be fair game.”
A Russian strike on supply points in eastern Poland or northern Romania would be a departure from recent norms. The United States may have carpet-bombed Laos and invaded Cambodia during the Vietnam War in hopes of smashing Viet Cong supply lines and sanctuaries, but the approach proved both politically costly and strategically ineffective.
Mr. Hoffman and others cite several examples pointing the other way, including the Soviet and U.S. wars in Afghanistan. In both cases, Soviet and American leaders feared the costs and potential consequences of expanding already difficult conflicts.
For Mr. Putin, the stakes would be high. With his army foundering, some analysts say a vigorous NATO supply campaign could lead to a Russian military failure, with potentially disastrous implications for him.
“When an insurgency has access to cross-border sanctuary and safe haven and a flow of arms, it is almost impossible to defeat,” Mr. Hoffman said.
Soviet leaders learned that lesson the hard way in the 1980s, after Moscow invaded Afghanistan and struggled to control the country amid fierce resistance by Islamist insurgents known as mujahedeen. A C.I.A. program soon turned western Pakistan into a staging ground for the mujahedeen fighters, eventually providing them with more than $2 billion in equipment, training and weapons — including Stinger antiaircraft missiles, which the United States is now shipping to Ukraine.
Pakistani officials worried that the Soviets might respond with cross-border attacks on their territory, but the United States assured them — correctly, as it turned out — that Moscow did not want to devote even more troops to an already unpopular war.
“The Soviets certainly had the military capability to strike the bases either through airstrikes or ground forces, or both,” said Bruce Riedel, a former C.I.A. analyst who is now with the Brookings Institution and wrote a book about the covert campaign to support the Afghan fighters. But, he added, “Moscow did not want the war to expand.”
Some analysts say that, in this case, Mr. Putin may lack the troops needed to complete a takeover of Ukraine, never mind to storm into or seal the border of a second country.
But Mr. Putin enjoys more sophisticated and accurate weaponry than his Soviet predecessors, making it easier to strike supply hubs for Ukrainian forces from a distance.
Mr. Putin may also doubt that NATO would treat a limited, lightning strike from the air on its territory as an act of war, especially given Mr. Biden’s warnings that direct conflict with Russia could lead to World War III.
Even so, some doubt that Mr. Putin would take such a risk.
One reason is that it is “very, very difficult for countries to seriously interdict supply lines,” said Seth Jones, a former official at U.S. Special Operations Command now with the Center for Strategic and International Studies in Washington. “Limited striking of targets in Ukraine, or in countries like Poland, is not going to meaningfully stop the flow.”
Mr. Putin could try a step short of overt military action. Within months of Russia’s 2014 intervention in eastern Ukraine, in support of pro-Moscow separatist rebels, two large explosions destroyed a pair of arms depots in the Czech Republic and Bulgaria.
Officials in both countries have since attributed the attacks to sabotage by Russia’s military intelligence arm, the G.R.U. The presumed motive was to prevent the weapons from reaching their destination: Ukrainian fighters battling the Russian-backed separatists.
In a detailed investigation of the events, the open-source news outlet Bellingcat said the attacks were “part of a longer-term G.R.U. operation aimed at disrupting Ukraine’s capabilities to procure weapons and munitions critical to its defense against Russian troops and Russia-sponsored militants in the war in eastern Ukraine.”
Russia-Ukraine War: Key Developments
The status of peace talks. President Volodymyr Zelensky said in an interview with Russian journalists that Ukraine was “ready” to discuss a neutral geopolitical status but insisted that he would not cede sovereignty. Ukrainian and Russian diplomats are planning to meet in Turkey.
On the ground. As the war entered its fifth week, Ukrainian forces appeared to make gains in the northeast. Despite talk of Russia focusing on the east, fighting across multiple battlefronts, including in the southern city of Mariupol, suggested a more dynamic and volatile situation.
Biden’s speech. During a speech in Warsaw, President Biden said that Russian President Vladimir V. Putin “cannot remain in power,” but U.S. officials scrambled to walk back the unscripted remark. In Europe, Mr. Biden’s comments were met with a mix of rejection and admiration.
As Biden officials try to predict Mr. Putin’s response to their arms supplies, they will be mindful of instances when the United States faced similar choices.
After the United States invaded Afghanistan, President George W. Bush never moved seriously against cross-border sanctuaries the Taliban had established in Pakistan. Sealing off those locations might have required doubling the number of U.S. troops in the country from around their peak levels of the war, according to Gian Gentile, a retired U.S. Army colonel and historian at the RAND Corporation. That was never in the cards.
“The Americans were not willing to escalate militarily,” said Husain Haqqani, a Pakistani ambassador to Washington during the Obama administration. Even when President Barack Obama ordered a raid on Osama bin Laden’s compound in Abbottabad, Pakistan, in 2011, Mr. Haqqani added, “a big concern was, what if Pakistan retaliates and this becomes some kind of full-blown war?”
In 2007, U.S. officials, including Mr. Bush himself, said that Iran was equipping Iraq’s insurgent Shiite militias with improvised rockets and deadly roadside bombs that featured an explosively formed penetrator, which could punch through most U.S. vehicles.
Senator Joseph I. Lieberman, an independent from Connecticut, was among those who called for a military response, saying that Iranian leaders could not “have immunity for training and equipping people to come in and kill Americans.” Mr. Bush, who already had two wars on his hands, never struck.
During the Korean War, the United States even shied away from bringing the fight to an enemy that was directly attacking its troops. After U.S. forces advanced near North Korea’s border with China in late 1950, Beijing sent hundreds of thousands of troops south to push the Americans back. Gen. Douglas MacArthur sought permission to bomb bases, bridges and factories in China. But President Harry S. Truman rejected the idea, heeding the advice of other commanders who said an expanded war could draw in the Soviet Union and leave the United States outmatched in the region.
The Soviets nevertheless sent fighter jets to help battle the U.S. Air Force over Korea. Although Moscow denied its involvement, even marking its planes in North Korean colors, the United States suspected the truth — but chose to look the other way.
Perhaps the most obvious counterexample is the Vietnam War. Two U.S. presidents, Lyndon B. Johnson and Richard M. Nixon, ordered major military operations beyond Vietnam’s borders, in Laos and Cambodia, to cut off the jungle supply routes and bases collectively known as the Ho Chi Minh Trail.
The impoverished rural nations could do little to defend themselves. And the operations carried a devastating punch. More than two million tons of bombs fell on Laos alone. But, Mr. Jones noted: “It didn’t work. America still lost that war.”
John Ismay contributed reporting.
