Australia and the United Kingdom are set to sign a landmark 50-year defence treaty, strengthening their alliance amid uncertainty over United States support for a key nuclear submarine program. The agreement, which underpins the existing security partnership between the two nations, is expected to boost economic cooperation and industrial capacity, particularly in the defence sector. As part of this collaboration, Australia will invest $5 billion to help British industry design and produce nuclear reactors for future submarines. The deepening ties come as the United States reviews its role in the trilateral security pact, prompting Australia and the United Kingdom to reaffirm their commitment through more frequent ministerial meetings and joint military exercises, such as the recent visit of the United Kingdom's Carrier Strike Group to Darwin.