President Obama today sent a letter officially notifying Congress that he had dispatched American troops - 100 Marines - to embassies in the Middle East.

Why notify Congress?

By law, the White House has to. Consistent with the War Powers Resolution, the president has to notify Congress if he/she dispatches combat-equipped troops to a foreign country.

Although these security forces are equipped for combat, these movements have been undertaken solely for the purpose of protecting American citizens and property. These security forces will remain in Libya and in Yemen until the security situation becomes such that they are no longer needed," he wrote in a letter to congressional leaders.

Read more about the Marines dispatched to Yemen and Libya.

In March of 2011, Obama was criticized by some for not notifying Congress about a much larger show of American military force in Libya.

Americans, as part of a NATO-led effort, provided planes, ammunition and flew predator drones over rebel-controlled areas like Benghazi as rebel forces sought to - and ultimately succeeded in - overthrowing Muammar Gaddhafi.

At the time in 2011, Republicans in Congress criticized the White House for not seeking congressional authorization for the U.S. involvement, which ultimately cost more than three quarters of a billion dollars.

Read more about U.S. involvement in the NATO-led No-Fly Zone in 2011.

How is today's action - 100 Marines - different from the earlier U.S. involvement in Libya - in which the president was criticized for not notifying Congress?

Very simple - these are boots on the ground. In that instance, no combat equipped troops were sent to a foreign country. Instead they were planes and drones based outside the country and flying over.

To recap: Predator drone strikes, combat flights and more than $770 million in taxpayer dollars need no congressional notification.

One-hundred Marines sent to protect embassies under duress does need congressional notification.