@GreyWolfIn my story Aztlan Portal, the readers soon get to see the Aliens, some of whom appear to be quite similar to humans, but others appear to be "Monsters" (but humanoid, two arms, two legs, one head, a torso). Local police, and ordinary people are of course taken by surprise. State authorities, for reasons of bureaucracy (No official wants to be the First to "scream" Monsters!) Law Enforcement and military forces are told they are dealing with rather serious "unrest" and believe they are merely dealing with humans, at worst members of Narco Cartels with "assault rifles" and maybe a few heavier weapons.
My story is "Hard"(ish) Science Fiction (the Portals alone are a stretch; the aliens and their society...). My story is set solidly in 2018, until events create an alternate timeline. However, historic events still have a major effect on the story; governments (and other powerbases) are focused on "Real World Problems" not some "Internet Hoax" in the Hinterland of Chihuahua...
Few people know much about the USA, UK, or Russian armed forces in the second decade of the 21st Century; let alone the Mexican armed forces, including the Federal Police ("Federales") who are configured to protect the government, in particular the Federal Mexican Government from the Narco Cartels, and to a lesser degree from "indigenous" or "radical" insurgencies. Not conventional or "peer" forces. My story is intended primarily to entertain, but also to inform my readers about Asymmetrical Warfare. I received quite a number of comments and complaints that "I was going into too much detail about the weapons, vehicles, and equipment of the Mexican forces. I was Showing (in a way "Chekov's Assault Rifle") not telling, that the Mexican forces were prepared to face the threat the Government Bureaucracy wanted them prepared to deal with. "Monsters" were difficult, if not impossible to Kill with the "vaunted" "AR-15" aka "Black Rifle" or M16 (and various other 5.56x45mm caliber weapons) could not "One-Shot-Stop" many of their opponents; all though some might eventually die of wounds and/or bleed out.
Often when people see events such as the US debacle in Afghanistan, they wonder "How could such a thing happen?" Could the USA have "won" in Afghanistan? Could Russia have captured Kiev in 72 hours, or a week or two? The seeds of victory or defeat are sowed long before we see the videos! In my story NORAD is aware of the initial incursion, although they lack significant information and context. Theoretically, if the USA and/or Mexican governments had investigated matters, and decisively committed forces (and logistics) they probably could have been successful in 30-45 days (requiring 15-30 days for preparation). Of course, No One with power wanted to propose to POTUS Trump committing the 1st Cavalry Division and the 4th Infantry Division, the 82nd Airborne Division, 75th Ranger Regiment and several Tactical Air Wings into Mexico.
What are some of the barriers to governments/bureaucracies responding effectively to asymmetric threats? Fear of "Optics" (bad "headlines" or TV images) "Looking Foolish" No one wants to be the first to cry "The Sky Is Falling!" perhaps the biggest reason is: "It'll COST TOO MUCH!" Until the Price of Not doing enough becomes clear. ...but hey, that Might be the Next Administration...
So, too few courageous Mexicans, and eventually a few personnel from the USA (and elsewhere) do the best they can under the constraints they must endure.
Nor are the Aliens without constraints too.
In my story various versions of "Chekov's Gun(s)" are all about. Bureaucrats are clear about Their priorities (Not the threats the readers may see). Responses are prepared that are preparing for the wrong things. Some, such as a wealthy Mexican business magnate, or some less influential government employees (as well as the poor bastards in the "frontlines"), have some inkling about what should be done.
https://storiesonline.net/s/25148/aztlan-portal