A bipartisan group of U.S. lawmakers is pressing the Army to explain why it told a unit based in Europe to stop specializing in drone warfare, an order that comes as the world's battlefields rapidly evolve and military tactics increasingly rely on uncrewed systems to fight.

The 173rd Airborne Brigade was building its own drones and practicing the kind of warfare that Ukraine has pioneered against Russia and that Iran has fought against the U.S. - warfare that has killed and wounded American troops.

The brigade of 600 soldiers was set up in November to be deployed anywhere that drones were needed.