Why is the US currently at war attacking ISIS targets in Syria?

I am sure you may have the answer to this and since you have the answer, you may want to move on to another article. Okay, there are the simple narratives that perhaps may be dismissed so fast and digested ones. It is your pick.

Don't consider:

• Golan Heights are resource with water that can serve Palestine, Israel, Jordan, Syria.
• The fact that Iran is supporting the Syrian Gov't.
• The fact that neorealist/neoliberalist  commonsense compel the powers that be to work with a legitimate state than rebels who are fighting against it
• The fact that this is a proxy war between USA and Russia

But, let us just remain simplistic and trace the history:

Syria is part of the Middle East and is bordered by Turkey in the North; Jordan is in the South; Mediterranean Sea is on her west; and Iraq is on the East with a population of about 18.0 million. 75.0% are Sunni and 13.0% are Shia. The current President Assad, belongs to a subgroup of Alawi Shia (World Population Review website). Ghouta, Douma and Hamouriya in Damascus, are rebel strong-holds e.g., Nusra Front, Jaish al-Islam, Faylaq al-Raman and Hayat Tahrir al -Sham. The war started in 2012 and has left 5.6 million in 1,244 communities in acute need (according to BBC). Syria has 6.5 million internally displaced people.

The civil strife compelled bodies such the UN, Red Crescent and Red Cross to play pacifying roles because the state has failed to provide legitimate quality life for her citizens. Different allies joined different warring sides to address different needs. The US is on the side of the Sunni while Russia is on the side of the Shia. The compulsion for the US to attack Syria is multifold: it establishes a US-led presence, ensures US interests are met, creates a buffer for responsible conduct, checks Syria’s defensive arming and counters Russia’s show of power in the region (Blanton and Kegley, pages 23-27). 

But.........

Because of the xenophobia experienced by many who fled to Europe and the fact that the Assad regime is gaining a semblance of order, Syrians are returning in droves. A state is one of the only entities which can assure citizens of anti-poverty programs, sanitation, steady clean water flow, a basis for quality life and opportunities for innovations to turn out to be thriving industries. This is a motivation for people to be sympathetic to stabler governments. When there are options to choose between Assad and ISIS it is any one's guess which side may be popular.

ISIS on the other hand has now turned into a pariah marketing mayhem and leaving a trail of disorderliness. So, matters have changed. Assad is the lesser of the devils and ISIS is the enemy in the region.

References:

Blanton, Shannon, and Kegley, Charles (2017). World Politics Trend and Transformations. 2016-2017 edition. Routledge

Population of Syria: http://worldpopulationreview.com/countries/syria-population/.

The BBC, March 11, 2016

Syria: The Story of the Conflict

The BBC, Feb 23, 2018

Syria war: UN Security Council truce vote delayed