system
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human
[MCU] What have the world militaries been up to lately? Did Tony Stark really privatize World Peace?

Sorry if this sounds long or rambling, I just wanted to post my observations and discuss the state of the world's different militaries as they stand in 2018 and beyond.

It seems like military branches in the MCU have less and less influence on the world as time goes on. During Iron Man 1 and 2, they get involved but kind of eventually let Tony Stark do his own thing. He has even gone as far as declaring that he has privatized World Peace. The US government played a big role in The Incredible Hulk, but have since stepped down and backed off. After The Avengers, we don't see much government stuff, as the focus is put onto SHIELD (Which is later shut down).

I know General Ross is in Civil War and Infinity War, but we don't really gain much understanding of what the military does, or how influential they are in this post-Avengers world. The Avengers pretty much still tackle every major threat, and the Army never exactly steps in to help. Where were all the tanks or soldiers in NYC during Infinity War? Where were they in Endgame? I acknowledge Thanos probably did a ton of damage to the army, but surely, there would be some kind of organization trying to keep things in order or enact Martial Law or something?
gpt
In the years since Captain America saved the world from Hydra, the world's militaries spent a significant amount of time and resources trying to recreate Erskin's super soldier serum. The end results were things like the Hulk, Black Widow, and a variety of other enhanced individuals. Governments witnessed the effect of deploying a single super-powered individual into combat zones and it changed the face of warfare in their world as much as the atom bomb changed ours. 

Given this sudden emphasis on intelligence and espionage over large scale conflict, government focus (and funding) went less into stockpiling nuclear arms and into designing and creating super soldiers and arming them appropriately. Of course, no one was really as effective as they would have liked to have been, but this new military landscape led to the rise of clandestine organizations like S.H.I.E.L.D. and facilities like the Red Room. It was in everyone's best interest to keep their inner workings locked down, since everyone was trying to copy everyone. And when dealing with super-powered individuals, mistakes tend to be... abominations. 

To answer your specific questions; 

The military's involvement in the Battle of New York was delayed because they had no idea what was going on. The National Guard was deployed because when reports started coming in that there was an Alien Invasion they assumed it was hysteria. The military officials that were involved and actually understood what was going on fired a nuke at the city. 

In Infinity war, there was no response team because there was no response time. By the time the military had mobilized, the fight was over.

Same thing with Endgame.  Strange and the sorcerers didn't open up a portal to a military base because they would have had to convince the soldier's to march through it. They needed a timely response to Thanos.