With live explosives, smoke and rocket fire from helicopter gunships, American troops and their international partners are trying to give Iraqi soldiers a realistic idea of what they can expect when they take on Islamic State militants.

Analysts say a variety of factors, from sectarian divisions to corrupt leadership practices, led to the collapse of the Iraqi army last year when Islamic State fighters swept across large parts of northern and western Iraq.

The disintegration of Iraq’s army was all the more striking because the U.S. had spent $25 billion rebuilding, training and equipping it after the Americans disbanded Saddam Hussein’s military in the wake of the 2003 U.S.-led invasion. […]