Which of these was the worst mistake made by a U.S. President?

[Lincoln Republican]

Which of these was the worst mistake made by a U.S. President?

Please vote and discuss should you wish.

[Lincoln Republican]

I had considered putting the Iraq War under G W Bush as one of the options, but decided against it as I  believed the Iraq War would dominate the voting, and would not leave an accurate reading of how the other issues were viewed. 

[Lincoln Republican]

What, no mention of Buchanan's letting the Union break apart in his final months in the White House?

I realize that the Civil War resulted in the tragic deaths of many thousands, an estimated 620,000.

But one could argue that the breakup of the union resulted in the abolishment of slavery.

The abolishment of slavery was one of the greatest and most momentous developments in the history of the United States, in fact, in the history of the entire world.

[Lincoln Republican]

What, no mention of Buchanan's letting the Union break apart in his final months in the White House?

I realize that the Civil War resulted in the tragic deaths of many thousands, an estimated 620,000.

But one could argue that the breakup of the union resulted in the abolishment of slavery.

The abolishment of slavery was one of the greatest and most momentous developments in the history of the United States, in fact, in the history of the entire world.

I might add that all the mistakes and scandals I have listed are from the twentieth century.