What happened in 2014 that made the ongoing civil war in Syria even more complicated?

(1 point)

The Arab Spring led to demonstrations in Syria.

The Assad government used chemical weapons.

ISIS launched an attack on Syria.

The United States attacked a Syrian air base.

All of the given options contributed to the increasing complexity of the ongoing civil war in Syria in 2014. However, the option that directly impacted the situation that year is: The United States attacked a Syrian air base.