Complex sentence: Meanwhile, the U.S. and Brittan were in the midst of highly intense negotiations begun in 1940 to establish a comprehensive intelligence partnership that as underway ad hoc and would be finalized in May 1943.
Split into simpler sentences: Meanwhile, the U.S. and Britain were in the midst of highly intense negotiations begun in 1940 to establish a comprehensive intelligence partnership. The partnership was underway ad hoc and would be finalized in May 1943.