The Messerschmitt Me 262 Schwalbe entered Luftwaffe service in mid-1944 as the first jet fighter to fly in combat. It was roughly 150 km/h faster than the best Allied piston fighters, and its performance advantage was genuine and unanswerable in a straight fight. It changed nothing about the outcome of the war.
What it could do
The Me 262 cruised above 800 km/h with four 30 mm cannon in the nose — an armament heavy enough to destroy a four-engine bomber with a short burst.
Against bomber formations it was extremely dangerous. Escorting fighters could not catch it, and the standard defensive tactics developed against piston fighters did not apply to something that closed at that speed.
Why it did not matter
Engines. The Junkers Jumo 004 was the first mass-produced turbojet, and it was built under a blockade that denied Germany nickel and chromium — the alloying elements a turbine needs. Engine life was around 25 hours, and often much less. Engines had to be handled gently: opening the throttle too quickly caused a flameout or a fire.
Numbers and fuel. Roughly 1,400 were built, but only a fraction were ever operational at once. Germany by 1944 had lost most of its synthetic fuel production to Allied bombing, and jet fuel and pilots were both scarce.
Vulnerability on the ground and in the circuit. The Me 262 accelerated slowly and was helpless on approach. Allied fighters simply waited near the airfields. A large proportion of Me 262 losses happened during takeoff and landing.
Pilots. By 1944 Germany was short of experienced pilots, and the aircraft demanded careful handling.
The delay question
A persistent claim holds that Hitler personally delayed the programme by insisting the aircraft be developed as a bomber, and that this cost Germany the air war.
The interference happened. Its importance is generally judged to be overstated. The binding constraint was engine metallurgy, and the Jumo 004 was not ready for reliable service any earlier regardless of what role was specified. Later analysis tends to conclude that the aircraft could not have been available in decisive numbers whatever the political decisions.
What it left behind
The Me 262's swept wing — adopted primarily to fix a centre of gravity problem rather than for aerodynamic reasons — turned out to delay compressibility effects at high speed. Captured examples and German research data went to the United States and the Soviet Union after the war, and swept wings appeared on the F-86 Sabre and the MiG-15, which met each other over Korea five years later.
The aircraft is a clear demonstration that a technological advantage is not the same as a military one. Germany had the better fighter and had already lost the war that would have needed it.