Complex sentence: U.S. funding for the Contras during their anti-Sandinista war was later found to have been derived from covert sources given the failure of the U.S. Congress to authorize sufficient funds for the Contra's efforts and the Boland amendment barring further funding.
Split into simpler sentences: U.S. funding for the Contras during their anti-Sandinista war was later found to have been derived from covert sources. The U.S. Congress did not authorize sufficient funds for the Contra's efforts, and the Boland amendment barred further funding.