There is a civil war in Yemen where American troops are sent to fight Al Qaeda. It is a conflict between the Houthi ethnic group that occupy a northern region of Yemen and they want autonomy from the federal government (Wikipedia).
Houthi activists accuse the government bureaucracy lack the proper expertise.
The civil war has spilled into Saudi Arabia’s territory, triggering military response.
The US is there because Al Qaeda fighters are helping to fight the official government elect.