What accusion was given by president bush as a reason for invading iraq but later proven false

That Saddam Hussein's regime in Iraq posed a major security threat because they possessed weapons of mass destruction.

Explanation:

The war in Iraq was pursued beginning in 2003 as a preemptive strike against the regime of Saddam Hussein and the perception that he had stockpiles of weapons of mass destruction that could be used for terroristic purposes. That perception did not turn out to be a full reality, but the war pursued regime change in Iraq nonetheless. Saddam Hussein was removed from power. Later, in 2006, Hussein was convicted of crimes against humanity by the Iraqi Special Tribunal. He was executed by hanging.

Saddam Hussein had weapons of mass destruction.