What branch of the U.S. Military would respond first to a HUGE crisis?

We’d likely see FEMA first.
First responders would be the local police and fire departments. Army reserve would probably move in first of the military groups.

Next would come the National Guard.

Natural disaster (ie hurricane) I’d say FEMA.
Terrorist/biological/military I’d say national guard as well as local responders like police/fire/paramedics.

I’m not sure how the US Military is divided but I imagine it’s not that different to the British Military

Army
Air Force
Navy

I would imagine it would be the Army as they generally have the largest man power and have the widest range of situational training.

After they ‘saved’ people I would imagine they would set up a quarantine area and wait for instruction from those that could determine the biological threat. If the infected were able to spread the disease easily I guess the most sensible action would be to keep them quarantined and let them die, or assist their deaths (take that how you will)

If those that were affected by the biological weapon posed no threat to the wider population I guess they would focus on getting them to treatment centres and securing the area against looters.

Actually, the first responders would be local police and firefighters and emergency medical personnel. They coordinate their efforts and drill for large-scale emergencies of different types.

If you’re looking at this from a military standpoint only, one of the first considerations could very well be the National Guard because they are partly tasked with assisting local and state agencies in dealing with emergencies that affect the population of their respective states. It could be the Army or Air National Guard, the Army or Air Force Reserve, or even Marine or Navy personnel that have reserve programs in certain cities and states.

Continuing on, the amount of manpower that an individual unit can contribute to assisting in the emergency comes into play and does that specific unit have service members trained to deal specifically with the crisis that is occurring? (A biological / chemical attack for instance) Whatever the situation may be, all branches of the Armed Forces have teams specially trained to respond, contain, and assist in various types of contingencies. Some branches would probably be better suited than others, depending on what needed to be done.

As far as quicker response times, more manpower, and more resources to draw upon, sometimes active duty bases are better in this regard because some national guard bases don’t operate 7 days a week and their mission requirements may not meet the standards of what will be expected in a crisis situation. A dozen different variables could come into play so there isn’t any easy answers when determining which branch of service would be most favorable, what assets they can offer, and how effective they will be in the crisis management and containment of an incident.

A biological attack would bring out the CDC wouldn’t it?!

@ShanEnri. That could be very likely and the military counterpart to the CDC, which is USAMRIID (U.S. Army Medical Research Institute of Infectious Diseases), might even end up playing a role at some point. Just like they did in the fictional but sobering movie called “Outbreak”.

I see, Thank you all so much for answering my question! ^_^ You all were very helpful to me!