Preceding World War I, it was almost a necessity for armies to secure the high ground, by overpowering their opponent atop a hill, in order to win battles. Getting to a higher location gave the army on top of the hill the advantage of shooting down on the opposing army, which had to advance up a hill with bullets raining down on it. Historically, armies with the high ground advantage have won more often than not.
The new high ground is space. The U.S. currently uses space in a passive way during combat, so let's look at space from this angle first.
In 1991, the United States and its allies used sophisticated satellite technology to pinpoint Iraqi targets during the Persian Gulf War. Intelligence-gathering satellites gave the American forces an unprecedented view of the battleground, showing every move that the Iraqi armies were making during the war. With the vast expanse of the desert landscape providing visibility, satellite imagery became the main source of information on the Iraqi army during the war.
Satellites also were a valuable tool in deploying troops during the Persian Gulf War. A constellation of satellites orbiting Earth, known as the Global Positioning System (GPS), was used by soldiers on the ground to determine their bearings. These 24 satellites gave the longitude, latitude and altitude of the U.S. soldiers carrying handheld GPS receivers on the battlefield. The open desert was an ideal location for using the GPS satellites, because there were very few natural objects around to interfere with the satellites' signals. In combination with the imagery from the spy satellites that were tracking enemy troops, the GPS gave the United States and its allies the advantage of knowing exactly where to position their troops for the greatest benefit.
The next frontier in space is much more active -- satellite weapons systems designed to shoot down nuclear missiles.
In May 1983, Reagan proposed his Strategic Defense Initiative (SDI), now termed Ballistic Missile Defense, which called for laser-equipped satellites to shoot down intercontinental ballistic missiles (ICBM). ICBMs have a range of more than 6,000 miles (10,000 km). At that distance, an ICBM fired from North Korea could easily reach Honolulu or Los Angeles. Reagan's SDI, also known as "Star Wars," was designed to provide an umbrella of protection from such missile attacks. The SDI satellites would track a missile from liftoff, and shoot it down with lasers before the missile cleared the air space of the country from which it was launched. Work on a space-based laser for Ballistic Missile Defense is proceeding despite some international criticism. The project has continued to receive $4 billion per year, and recently had an extra $6.6 billion budgeted for the project through the year 2005.