Soon after planes were first used in war, there were specialists on the ground coordinating strikes. Space-based weapons could one day yield new observers.
The U.S. military is sending an aircraft carrier to the waters off South America, in the latest escalation and buildup of military forces in the region.
The early warning radars on Greenland are key to defending the U.S. against nuclear attack, but they're vulnerable to hypersonic missiles, critics warn.
Putin emphasized that the maneuvers had been planned in advance, but they came shortly after the prospects of a meeting with Donald Trump fell through.