James Buchanan ( April 23 , 1791 - June 1 , 1868 ) was the 15th President of the United States . James Buchanan , Jr. ( April 23 , 1791 - June 1 , 1868 , bjuË Ë kÃ nÉ n ) was the 15th President of the United States ( 1857-1861 ) .