Tuesday marks a turning point in US involvement in Iraq.

It’s the deadline for US troops to withdraw from Baghdad and urban areas and hand over security there to Iraqi forces.

It comes as the US military says four American soldiers were killed in Baghdad on Monday. Iraqis celebrated the milestone in Baghdad. There was singing and cheering and impromptu parties as workers planted Iraqi flags in full view.

By Tuesday’s deadline and in a big ceremony, US troops literally handed over the keys to a Baghdad base. It's the last of scores of them now under Iraqi control

President Obama agreed to the shift for US troops, a step toward combat withdrawal next year. The US’s bet is that Iraqi forces can handle urban security. They now man Baghdad's checkpoints.

As Iraqis showcased their training, attacks have spiked leading up to Tuesday’s turnover: Last week in bombings in mainly Shiite areas 250 people were killed.

In response, the government banned motorcycles used in three of them. Experts say Iraqi troops have made strides. Some US troops will stay in cities advising Iraqi forces, ready to call in reinforcement if needed.

For now all 131,000 US troops in Iraq will stay. The administration says combat forces will start to come home early next year. It says all of them will be back by August. Defense experts call that a bigger test for Iraq.