For decades, right-wing extremists and jihadists have been responsible for the bulk of America’s deadly political violence and domestic terrorism, from the 1995 Oklahoma City bombing to the 2016 Pulse nightclub shooting, according to researchers.

       But those tracking such violence say more acts are being committed by those holding either explicitly leftist views, no dominant political views or a mix of fringe ideologies, including the alleged gunman in the fatal shooting of a UnitedHealthcare executive in December and the suspect in the killing of two Israeli diplomats in Washington in May.

       This Payday invest the right way!

       Save Big with Best Deals!

       Subscribe now

       Already subscribed? Login

       Premium benefits

       200+ Premium Articles Weekly

       WSJ, The Economist Access at < INR 340/month

       Build Smart Finance Plans with Experts

       Top Stock Recommendations Everyday

       Expert-Backed Investment Growth

       Access Mint Lounge, Newsletters & Insights

       WARSAW—Drones darting across NATO airspace have exposed holes in the defenses of an alliance built for more traditional military conflicts.

       The North Atlantic Treaty Organization, created to counter Soviet tanks and warheads, is increasingly confronting new and different threats—nontraditional ones that include sabotage, cyberattacks and drones.

       NATO and its member countries are accelerating efforts to respond to such threats after drones appeared in the skies over Poland and Denmark. In Poland, NATO warplanes shot down several Russian drones earlier this month.

       Then, days later, drones appeared again—not near the combat zone in Ukraine but above Danish airports, disrupting flights. The aircraft disappeared before authorities could intercept them. Danish authorities said they didn’t know who was behind the drones but that they appeared to be the work of a professional actor.

       Danish Prime Minister Mette Frederiksen said Thursday on X that she had spoken to NATO Secretary-General Mark Rutte about the incidents and they would examine “what we can do together to ensure safety and security."

       The incidents have brought fresh focus to how the alliance responds to drones and other nontraditional threats. NATO is now racing to adopt new technologies that can spot UAVs from afar and provide inexpensive defenses for use by both militaries and police forces. The alliance is also grappling with touchy issues, including rules of engagement for automated weapons and how to handle airspace incursions when intentions can’t be determined.

       Polish officials say they expect more incursions from Russia. Drones that were recovered earlier this month had been modified with additional fuel tanks to help them fly farther, indicating Moscow deliberately sent them through Ukraine, into Poland, officials there said.

       After that drone incursion, NATO’s Supreme Allied Commander Europe, U.S. Gen. Alexus Grynkewich, mobilized alliance forces under the banner Eastern Sentry to prepare for further spillover from Russia’s war in Ukraine or in other locations.

       Addressing the new threat presents a conundrum for NATO, which defends its 32 members but doesn’t have the authority to take action everywhere. Airport disruptions, for example, generally fall under the jurisdiction of interior ministries or homeland security authorities, not militaries.

       As the war in Ukraine has shown, defending against drone attacks is difficult and expensive. Uncrewed aerial vehicles are generally too small, slow and low-flying to be detected by existing military radar systems. Weapons that defenders might use to shoot down UAVs, such as Patriot missile-interceptors, cost far more per shot than drones do, and projectile supplies are limited.

       When Russia sent the wave of drones into Poland, NATO planes scrambled to intercept them but stopped short of using pricey rockets to shoot down all of the styrofoam-and-plywood UAVs. NATO officials say they assume Moscow wants to force them into making difficult choices and waste resources.

       Emerging threats were on the agenda when NATO members this summer agreed to more than double their military spending, including a portion potentially devoted to homeland security.

       Rutte said on Tuesday that Grynkewich “has the tools and authorities he needs to ensure our defense." Rutte said that Eastern Sentry “adds further strength and flexibility to our posture along our eastern flank—and anywhere else we might need it."

       Still, the war in Ukraine has given Russia more than three years of experience in multidimensional war that combines hybrid activities meant to disrupt civilian life with a military approach that employs mass barrages of cheap and expendable artillery and drones to overwhelm enemy defenses.

       NATO, meanwhile, has largely continued preparing to deal with the type of threats for which the alliance was created: large conflicts decided by big weapons and massed forces. Drone warfare has only recently become a priority for alliance members.

       “The biggest problem right now is that Europeans are so focused on the high end of the conflict that they don’t do enough on the low end of the war," said Konrad Muzyka, director of Rochan Consulting, based in Gdansk, Poland, which analyzes the Russian military. “The Russians have been learning and a big chunk of what the Russians will be doing in the next war will come from the low end."

       U.S. and other NATO forces are now testing counterdrone technologies for battlefield use, but efforts are just beginning. Protecting civilian infrastructure from drones will take longer and cost far more. Alliance and European Union officials plan Friday to discuss ideas for a “drone wall" along some EU borders, potentially including electronic sensors and inexpensive interceptors.

       Early this year, after European governments suspected Russian-piloted ships damaged cables under the Baltic Sea, the alliance established Baltic Sentry, which combined new naval patrols of the busy waterway with increased use of subsea drones and sensors to detect activity. Grynkewich said in announcing Eastern Sentry that since Baltic Sentry began, no new attacks on local undersea infrastructure had been detected.

       The new Eastern Sentry aims to follow a similar model. NATO was able to stand it up very quickly because, like Baltic Sentry, it is deemed an “enhanced vigilance activity" under the Supreme Allied Commander’s existing remit, and not a new operation, which would require unanimous assent of all members.

       Member countries are also working to prepare. Germany’s armed forces this week are conducting a three-day exercise in the center of Hamburg, including a hypothetical drone attack on NATO troops moving through the northern port city on their way east. The maneuvers include practice intercepting drones using jammers and nets, according to Germany’s military.

       Unauthorized drone flights over Hamburg’s barracks and port are common, but none have been intercepted, an official told German media.

       Germany’s military is only allowed to shoot down drones over its facilities, while the police are entitled to do so everywhere else. Civilians aren’t allowed to interfere with drones in any way, including by jamming their signals. Germany’s northern states, whose sea ports are crucial for trade and energy supplies, are working to develop an antidrone strategy.

       Write to Thomas Grove at thomas.grove@wsj.com and Daniel Michaels at Dan.Michaels@wsj.com