Project Excalibur was a Lawrence Livermore National Laboratory (LLNL) Cold War–era research program to develop an X-ray laser system as a ballistic missile defense (BMD) for the United States. The concept involved packing large numbers of expendable X-ray lasers around a nuclear device, which would orbit in space. During an attack, the device would be detonated, with the X-rays released focused by each laser to destroy multiple incoming target missiles. Because the system would be deployed above the Earth's atmosphere, the X-rays could reach missiles thousands of kilometers away, providing protection over a wide area.

Anti-ballistic missile (ABM) systems of the time only attacked the enemy nuclear warheads after they were released by ICBMs. As a single ICBM could carry as many as a dozen warheads, dozens of defense missiles were required per attacking missile. A single Excalibur device contained up to fifty lasers and could potentially destroy a corresponding number of missiles, with all of the warheads still onboard. A single Excalibur could thus destroy dozens of ICBMs and hundreds of warheads for the cost of a single nuclear bomb, dramatically reversing the cost-exchange ratio that had previously doomed ABM systems.

The basic concept behind Excalibur was conceived in the 1970s by George Chapline Jr. and further developed by Peter L. Hagelstein, both part of Edward Teller's "O-Group" in LLNL. After a successful test in 1980, in 1981 Teller and Lowell Wood began talks with US president Ronald Reagan about the concept. These talks, combined with strong support from lobbyists at the Heritage Foundation, helped Reagan ultimately to announce the Strategic Defense Initiative (SDI) in 1983. Further underground nuclear tests through the early 1980s suggested progress was being made, and this influenced the 1986 Reykjavík Summit, where Reagan refused to give up the possibility of proof-testing SDI technology with nuclear testing in space.

Researchers at Livermore and Los Alamos began to raise concerns about the test results. Teller and Wood continued to state the program was proceeding well, even after a critical test in 1985 demonstrated it was not working as expected. This led to significant criticism within the US weapons laboratories. In 1987, the infighting became public, leading to an investigation on whether LLNL had misled the government about the Excalibur concept. In a 60 Minutes interview in 1988, Teller attempted to walk out rather than answer questions about the lab's treatment of a fellow worker who questioned the results. Further tests revealed additional problems, and in 1988 the budget was cut dramatically. The project officially continued until 1992 when its last planned test, Greenwater of Operation Julin, was cancelled.