"There are three key reasons why James K. Polk deserves recognition as a significant and influential American president," Polk biographer, Walter Borneman, wrote. "First, Polk accomplished the objectives of his presidential term as he defined them; second, he was the most decisive chief executive before the Civil War; and third, he greatly expanded the executive power of the presidency, particularly its war powers, its role as commander-in-chief, and its oversight of the executive branch." President Harry S. Truman summarized this view by saying that Polk was "a great president. Said what he intended to do and did it." The ramifications of Polk's presidency resound down to our own day. The most controversial action he took was the invasion of Mexico. Criticism by a young congressman by the name of Abraham Lincoln's hurt Lincoln's re-election to the congress. Polk was a fervent believer in Manifest Destiny and is the president considered most influential in expanding the U.S. across the continent. He was a lifelong slave-holder and was considered by abolitionists to be a tool of the pro-slavery powers.