A Ukrainian ground robot in testing.Via WarTranslatedA Ukrainian national guard brigade just orchestrated an all-robot combined-arms operation, mixing crawling and flying drones for an assault on Russian positions in Kharkiv Oblast in northern Ukraine.We are talking about dozens of units of robotic and unmanned equipment simultaneously on a small section of the front, a spokesperson for the 13th National Guard Brigade explained.It was an impressive technological featand a worrying sign of weakness on the part of overstretched Ukrainian forces. Unmanned ground vehicles in particular suffer profound limitations, and still cant fully replace human infantry.That the 13th National Guard Brigade even needed to replace all of the human beings in a ground assault speaks to how few people the brigade has compared to the Russian units its fighting. The 13th National Guard Brigade defends a five-mile stretch of the front line around the town of Hlyboke, just south of the Ukraine-Russia border. Its holding back a force of no fewer than four Russian regiments.Thats no more than 2,000 Ukrainians versus 6,000 or so Russians. The manpower ratio is roughly the same all along the 800-mile front line of Russias 34-month wider war on Ukraine. Russian troops still greatly outnumber Ukrainian troops, despite the Russians suffering around twice as many casualties as the Ukrainians since February 2022.The Ukrainian operation involved remote-controlled flying surveillance and minelaying drones, one-way explosive robots on the ground and in the air as well as gun-armed ground bots.In what amounted to a smaller-scale proof of concept for the recent combined-arms robot assault, a Ukrainian ground robot cleared a Russian trench in Kursk Oblast in western Russian back in September. Russia has attempted small-scale ground bot assaults of its own, but less successfully.The problem, of course, is that while robots are adept at surveilling and attacking, theyre terrible at holding. To hold ground, armies put infantry in trenches. They sit, watch, wait and call for reinforcements when the enemy attacks. Its tedious, taxing duty that requires constant vigilance.Constant vigilance is difficult when a human operator is remotely observing the battlefield through the sensors of a maintenance-hungry ground robot.Machines break down. And their radio datalinks are highly susceptible to enemy jamming, as the California think-tank RAND discovered when it gamed out a clash between hypothetical U.S. (Blue) and Russian (Red) army battalions partially equipped with armed ground drones. Blues ability to operate was degraded significantly by Reds jammers, RAND concluded.Its not clear the 13th National Guard Brigade even tried to hold the Russian positions it cleared in the all-robot attack.After nearly three years of war, Ukraine is arguably the worlds leader in military robotics. But the Ukrainians innovation is, in part, an answer to its desperationthat is, its struggles to recruit enough human soldiers to match the Russians person-for-person.MORE FROM FORBESCORRECTION (12/22): This article has been updated to note Kharkiv Oblast is located in northern Ukraine.