Complex sentence: Thatcher gave strong support to President Bush in reversing Iraq's invasion of Kuwait in 1990, and the resulting Gulf War (1991), to which she authorized 45,000 troops; they saw combat under her successor Prime Minister Major in Operation Granby.
Split into simpler sentences: Thatcher gave strong support to President Bush in reversing Iraq's invasion of Kuwait in 1990, to which she authorized 45,000 troops. In the subsequent Gulf War (1991), they saw combat under her successor Prime Minister Major in Operation Granby.