“You cannot dictate the rules of engagement,” Mr. Maple said. “If the enemy is going to do something, then you have to do something as well. No matter what rules you put in place, in a conflict situation the rules will go out the window.”

Defense contractors, identifying a new source of revenue, are eager to build the next-generation machinery. Last year, Boeing reorganized its defense business to include a division focused on drones and other unmanned weaponry. The company also bought Aurora Flight Sciences, a maker of autonomous aircrafts. Other defense contractors such as Lockheed Martin, BAE Systems and Raytheon are making similar shifts.

Mr. Maple, who has worked in the field for over four decades, estimates military spending on unmanned military vehicles such as drones and ships will top $120 billion over the next decade.

No completely autonomous weapons are known to be currently deployed on the battlefield, but militaries have been using technology to automate for years. Israel’s Iron Dome air-defense system automatically detects and destroys incoming rockets. South Korea uses autonomous equipment to detect movements along the North Korean border.

Mr. Maple expects more collaboration between humans and machines before there is an outright transfer of responsibility to robots. Researchers, for example, are studying how aircrafts and tanks can be backed by artificially intelligent fleets of drones.

In 2016, the Pentagon highlighted its capabilities during a test in the Mojave Desert. More than 100 drones were dropped from a fighter jet in a disorganized heap, before quickly coming together to race toward and encircle a target. From a radar video shared by the Pentagon, the drones look like a flock of migrating starlings.

There were no humans at the controls of the drones as they flew overhead, and the machines didn’t look much different from those any person can buy from a consumer-electronics store. The drones were programmed to communicate with each other independently to collectively organize and reach the target.