system
Given the following post, respond as if you're commenting on Reddit:
human
[World War Z novel] What was the status of the United States Marine Corps during the duration of the American campaign?

The Navy, Air Force, and presumably Coast Guard were all grounded since Secretary Sinclair didn’t believe that it was worth it trying to fuel, maintain, and arm a billion dollar warship or multi-million dollar fighter jet. The Army ended up taking the brunt of the march to New York. 

But what about the Marine Corps? What was the role and status of the Marine Corps during the War? A couple of Marine combat engineers made E-Tools but was that really all we did?
gpt
The United States Army saw massive reorganization throughout the war. Early on in the war the "Alpha Teams" were made most likely of elite marine units. While those teams were successful once the outbreak in New York got out of control the Alpha teams were unable to keep the outbreaks under wraps.

 After the battle of Yonkers which was primarily the Army (but probably had some marine units) the U.S. defense force fell apart and a there was a massive exodus to the west. This is probably where most marine units ended up disorganized, lost or dead. Once the west coast was secured the armed forces had to completely evolve, as you mentioned this meant no substantial machines would be used. In the novel they mention that they had to walk across the country. 

The Marines that were left were adopted into this new armed forces. The modern marines love their machines and that's not what World War Z was won by. That said the steady trigger finger and great discipline lends itself to the new methods of warfare quite well. Many army units were augmented with marine veterans.