WASHINGTON — On Saturday, President Donald Trump held a hideously expensive military parade in Washington, D.C., on his birthday. Trump and his top officials stood on a stage at the National Mall behind two tanks, before two large digital American flags. Military bands and troops, some on horses, some in vehicles, some in tanks, others in Howitzers, marched in the streets. So did a few robot dogs. An army parachute team jumped down. Helicopters flew over. Drones flew by. There were many, many tanks.