When the U. S. invaded Iraq: A. The U. S. had the full support of the United Nations
B. The U. S. spent years fighting the Iraqi army on battlefields
C. The U. S. was unprepared for the tensions between the Sunni and Shia
D. The Iraqi army initially won several battles

A. The U.S. had the full support of the United Nations

Explanation:

And not only did they support the United Nations Security Council, but many UN members took part in actions led by the United States. After Iraq occupied Kuwait, all attempts at diplomatic negotiations failed. The United Nations has, on several occasions, tried to resolve the crisis peacefully, but Iraq has not backed down. Therefore, a war offensive was launched to expel Iraqi troops from Kuwait and the bombing of Iraqi military and civilian capacities. In just a few days, a US-led coalition managed to defeat Iraqi forces. This episode in history is known as the Gulf War.