You really think a well trained army has not invaded the U.S. because a small percentage of citizens with no military training or skills is armed with some pistols?? it is because of the U.S. military not asshat citizens. Is the Mexican cartel afraid of U.S. forces or Bubba Joe with a rifle?
Oh.... brief history lesson... google War of 1812 as your teacher might have skipped that one

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You may want to read what Tony stated and my follow-up as a reply to Tony about battles on American soil
or against the infrastructure of America. In addition, I was referring to
modern wars since the revolutionary war.
With that said, typically a modern army knows that a well-armed militia is not a threat. In contrast, the government/politicians typically believe their armies can be tied down or beaten by a well-armed militia. It's enough fear to prevent them from sending in their military or encouraging too much government involvement that hinders a foreign invading army.
Today, not 1812, a well-armed militia has the ability to retaliate (invade) another country that initiated a war on American soil. We have a more diverse country that consists of much more than just gun owners willing to take the fight to another country if necessary...invading that country without needing to set foot in that country.
Simply, you can defeat a country
prior to invading them (putting ground troops on their soil) with our military (e.g. psychological warfare, biological warfare, political division, crypto-currency, hackers et cetera)...the U.S. refer to it as
defensive measures only, and its multi-agency units that are not exclusive to the military.
- Yet, the U.S. military has divisions devoted to such and we have civilians working with the military with the equipment to do the same. Simply, it's not just about guns.
For example, last summer my son and I visited several top universities in the world that he's considering attending for college.
Two of those universities in the United States
bragged during summer orientation visits that
they had incidents that required the local fire department, police, and the FBI because a few of the past students (now alumni) had built a
nuclear device in their dormitory room.
Further, at the other university, a few students (grew/isolated) a
dangerous biological virus in their dormitory room. Much of the equipment at both universities was purchased on eBay by the students to help prevent the detection of what they were building/designing.
- We sat in the front roll at those university summer orientation visits, I turned around and saw other prospective students...their eyes light up as if they walked into a candy store when they heard those stories from the university speaker.
In contrast, some of the parents were shaking their heads in disbelief with faces as if they were about to make a mistake if they allow their teenager to attend that university.
Today, students at another university that we visited last year...those students were involved in hacking into the finances of a foreign adversary.
Simply,
today, the defeat of any foreign army invading the United States does not need to be initiated by our military because we have too many creative (mad) scientist students at our universities and they do
not need guns regardless of the fact they may accidentally wipe out some of their university classmates while defending America.
Thus, I do think the Mexican Cartel should fear well-armed civilians
not using guns.
wrbtrader