The future of warfare felt a lot like playing a video game. Soldiers fastened on virtual-reality glasses and then moved their fingers across the joystick in their palms. A small drone buzzed and lifted in response. At a military base in Texas last month, American soldiers trained on how to operate small quadcopters, the kind that now dominate the battlefield in Ukraine and are increasingly the weapon of choice for combatants around the world. With an explosive attached, a drone costing...