If an enemy wanted to invade, take over or destroy the USA, the old school methods of vast quantities of raw man power, guns, tanks, and missiles thrown onto battle fields would not be a winning strategy.
In this new age, our Achilles heel is our enlightened freedoms and our obese capitalism.
If an enemy chose, he could use several nonlethal tactics that would not annihilate the population nor would it destroy infrastructure, property or buildings.
One approach would be to reduce or eliminate our ability to feed ourselves by forcing farmers out of business , buying up farm land , and turning the land into housing developments. Another approach could be to eliminate our ability to fuel ourselves by phasing out fossil fuel production by scaring us with misconceptions about climate change. Of couse, it would help our advisary to dumb down our population through inadequate education, and changing curriculums to deny our history. While we are at it, we could stop teaching how our government was designed to work. Why not stop teaching about the constitution and the declaration of independence too? Opening our borders to an invasion by simply turning a blind eye to the influx of illegal immigrants would be a good tactic. Destroying and rewriting our history by tearing down monuments and renaming institutions would be a good tactic. Reducing or eliminating our manufacturing capabilities by shipping the methods of production offshore and overseas would be a good tactic. Allowing crime to ravage our cities by defunding and demeaning our law enforcement would be a good tactic. Permitting potent new drugs to flood into our country along with legalizing the super strong varieties of the "new strains" of cannabis and destroy the minds of the next generation would be a good tactic. Force us to support foreign wars with money we do not have (and must print out of thin air) causing runaway inflation would be a good tactic. Let's not forget top deplete our strategic petroleum reserve and reduce our military readiness . Anything that worsens our economy will force more discontent amongst the middle class causing more strikes and inevitably higher prices.....a vicious cycle. An enemy would certainly want to control the media thus controlling the information we use to make decisions about our leadership. An enemy would see the value of social media in brainwashing our population. An enemy would want to influence our elections. An enemy would do everything in their power to make us think we are a bad people who earned nothing and stole from others.
Of course, an attack on our way of life as described above would not happen overnight. It would take a generation or more. It reminds me of the frog in the pot of water. The poor frog does not realize that the heat is on until the water is boiling, and by then, it is too late.
It is "my take" that we are the frog. The water has reached a simmer and is about to get to a full boil.
Only strong leadership can turn this around.