On February 6, 1778, France and the United States signed the Treaty of Amity and Commerce regulating trade between the two countries, followed by a defensive military alliance against Britain, the Treaty of Alliance. In return for French guarantees of American independence, Congress undertook to defend their interests in the West Indies, while both sides agreed not to make a separate peace; conflict over these provisions would lead to the 1798 to 1800 Quasi-War. Charles III of Spain was invited to join on the same terms but refused, largely due to concerns over the impact of the Revolution on Spanish colonies in the Americas. Spain had complained on multiple occasions about encroachment by American settlers into Louisiana, a problem that could only get worse once the United States replaced Britain.