BAGHDAD, Oct 19 (Reuters) - A senior U.S. army officer in Iraq was sentenced to two years in prison on Friday after a court martial found him guilty of illegally possessing thousands of secret military documents. Lieutenant-Colonel William Steele, 52, was acquitted earlier on Friday on the more serious charge of aiding the enemy, which carried a sentence of life imprisonment, for allowing security detainees to use his mobile telephone for unmonitored calls.