What is Act Of State Doctrine?

Legal Definition
The Act of State Doctrine states that every sovereign state is bound to respect the independence of every other sovereign state, and the courts will not sit in judgment of another government's acts done within its own territory.

The doctrine is not required by international law (neither customary international law nor treaty law), but it is a principle recognized and adhered to by United States federal and state courts. In the United States, the rationales for the doctrine include respect for other nations' sovereignty and protection of the U.S. Executive's prerogative in foreign affairs, both of which may be frustrated by a decision issuing from U.S. courts.

The Act of State Doctrine enters consideration most often in cases where a foreign sovereign has expropriated the property of a U.S. national located in that foreign territory (e.g. through nationalization). Rather than pursuing recourse through the courts, United States nationals are to take their claims against foreign sovereign governments to the Executive so that the government can either espouse the claims of all U.S. nationals as a group or seek recourse through diplomatic channels. The United States employs the Act of State Doctrine more broadly and with more frequency than other countries.
-- Wikipedia