John Jay spend most of the time during revolutionary war in Spain as a diplomat. He spent 3 year there garnering support for American Colonies in the Independence War. His mission was gaining financial support for Colonies and an alliance with Spain which entered the war on French side against Great Britain.
He also participated in Treaty of Paris in 1783 alogn with Benjamin Franklin and John Adams for negotiating with Britain the American independence.
He also helped the United States secure all land east of the Mississippi River, with the exception of British territories in Canada and Spanish territory in Florida, effectively doubling the size of the new nation.