The British conducted seizures of American shipping to search for British deserters from 1806–07; American citizens were thus impressed into the British naval service. In 1806, Jefferson issued a call for a boycott of British goods; on April 18, Congress passed the Non-Importation Acts, but they were never enforced. Later that year, Jefferson asked James Monroe and William Pinkney to negotiate with Great Britain to end the harassment of American shipping, though Britain showed no signs of improving relations. The Monroe–Pinkney Treaty was finalized but lacked any provisions to end the British policies, and Jefferson refused to submit it to the Senate for ratification.