The United States has not had a new branch of military service since 1947.
On Aug. 1, 1907, the U.S. Army Signal Corps established a small Aeronautical Division to take "charge of all matters pertaining to military ballooning, air machines and all kindred subjects." Military Times History
The Signal Corps began testing its first airplane at Fort Myer, Va., on Aug. 20, 1908, and on Sept. 9, Lt. Thomas E. Selfridge, flying with Orville Wright, was killed when the plane crashed. He was the first military aviation casualty. After more testing with an improved Wright Flyer, the Army formally accepted this airplane, identified as "Airplane No. 1," on Aug. 2, 1909.
The National Security Act of 1947 became law on July 26, 1947. Following World War II, the Act became a major re-write of national security creating the Department of the United States Air Force (USAF) and the Central Intelligence Agency (CIA and the National Security Council (NSC).
From 1947 National Security Act Section 208 (f)
In general the United States Air Force shall include aviation forces both combat and service not otherwise assigned. It shall be organized, trained, and equipped primarily for prompt and sustained offensive and defensive air operations. The Air Force shall be responsible for the preparation of the air forces necessary for the effective prosecution of war except as otherwise assigned and, in accordance with integrated joint mobilization plans, for the expansion of the peacetime components of the Air Force to meet the needs of war.
In the late 20st Century it’s became necessary for the USAF to protect space satellites. There are over 159 military satellites orbiting Earth, the most sophisticated being the Space Based Infrared Systems (SBIRs).
The $1.2 billion USAF SBIRS GEO Flight 4 satellite launched aboard the Atlas 5 rocket will finish the initial deployment of the Air Force’s new-generation missile-warning network. When complete, the constellation that will include a minimum of four SBIRS craft stationed in geosynchronous orbit and at least two infrared payloads in elliptical orbits aboard top secret National Reconnaissance Office spy satellites, providing polar coverage. Source
Note: GEO signifies the rockets are in geosynchronous stationary orbit of 22,300 miles above Earth.
Recently, a new department has joined the military establishment – the Department of Space Force. It will be a separate but equal military branch dedicated to space security, including the protection of U.S. interests in space.
The number of satellites worldwide has increased tremendously. Now every continent has satellites orbiting Earth. While many are commercial and security/intelligence satellites, some are dangerous, offensive warfare satellites.
1966 Worldwide Satellite Map
2016 Worldwide Satellite Map
In my opinion, the Department of Space Force has been a long time coming. With the growing nuclear space threats from enemy countries, it would be irrational of us not to protect our country and our allies from such malevolent forces.
Some countries already have the ability to make war through the use of space satellites. The current United States Air Force has a branch that already is doing their best to protect us in space. But the new Space Force will be a dedicated agency with the specific purpose of defense in this arena.