The first thing Obama should've done in office was to announce we would not tolerate the continued toleration of slavery in Mauritania. 20% of the entire population is held in slavery and in spite of a law passed in 2007 against the practice there has not been a single prosecution, yet people have been prosecuted after protesting against slavery. Hence, it is commonly understood in Mauritania that slavery is in practice still legal.
Since Mauritania refuses to do anything to stop the practice of slavery the United States and other countries that have abandoned the practice and are firm in their commitment to the abolish of slavery should invade Mauritania and stop slavery there.
This should take precedence over any intervention in Syria. As wrong as chemical weapons are Assad would've otherwise just killed civilians with regular weapons (not to mention evidence of rebels using chemical weapons too).
For there to still be any countries that allow slavery in the 21st century is a travesty and an injustice! That should be given higher priority than stopping chemical weapons.
Mauritania claims to have made slavery illegal in recent years, but the (few) stories that come out of there suggest otherwise. It's awful, it's disgusting, but it in most cases it isn't an issue of mortality. In Syria, people are being targeted and murdered because they want a leader that isn't a murderer. That is an obviously problematic situation that needs to be addressed more immediately.
It won't be because nobody on this earth can stop a problem anymore, but it should be.