Complex sentence: The British government had trusted that America would continue to share nuclear technology, which the it saw as a joint discovery, but the Atomic Energy Act of 1946 (McMahon Act) ended technical cooperation.
Split into simpler sentences: The British government had trusted that America would continue to share nuclear technology, which the it considered to be a joint discovery. The United States Atomic Energy Act of 1946 (McMahon Act) ended technical cooperation.