The Republic of Vermont banned slavery in its constitution of 1777 and continued the ban when it entered the United States in 1791. Through the Northwest Ordinance of 1787 under the Congress of the Confederation, slavery was prohibited in the territories north west of the Ohio River. By 1804, abolitionists succeeded in passing legislation that ended legal slavery in every northern state (with slaves above a certain age legally transformed to indentured servants). Congress banned the international importation or export of slaves on 1 January 1808; but not the internal slave trade.