Geographically the USA is in an enviable, virtually impervious position.  We are isolated from the majority of the world by oceans at least 3000 miles wide (the ultimate moat, 2 miles deep and filled with sharks).  We have only two borders.  We share one, to the north with Canada, an ally for well over 100 years.  If an attack came from the north it has to come across the Arctic Ocean, through 2000 miles of frozen Canada to get here, that aint happening.  Our border to the south, similarly buffered by 500 miles of desert on each side, we share with Mexico, an enemy in the past, currently an ally.  It is doubtful that the Trump regime will change this, our fates are intertwined.

How would an enemy attack us?  If they nuke us, the land would be unsuitable for them to occupy and exploit.  If they bomb us conventionally they might win, strategically speaking, but they would still have to cross the Oceans to invade.  Even if they take DC, they still would need to penetrate 1500 miles deep into the interior from each coast (to our bread, beef, dairy and potato basket in ID, KS, NE, OK, CO, IA, WI and TX) for total victory.  They also will want to acccess our fresh water supply (we have 75% of the planets supply), it is in the Great Lakes.  How to do that?

The Vikings showed us how in 886 AD, the British attempted it in 1812, and we toyed with it in Vietnam. Use the river systems.  In 886 the Vikings sailed up the River Seine to Paris and invaded, the effect of which was driving the center of European power east for 1000 years (Frankish Merovingian/Carolingian to Germanic HRE).  The British tried it in 1812, against the USA, entering the Mississippi River at New Orleans, unfortunately they did not get the word, the war was over.  Communication was still very slow.  Not even radio or telegraph, yet.
    

Traveling up the Mississippi one could reach, Baton Rouge, Memphis, St Louis, Chicago (and thus the Great lakes), Pueblo CO, Rockford,IL, Des Moines IA, Madison, WI, and, points farther west out the Missouri River right to the Rockies. etc.  Attacking from the west will not work, you get stuck at the Rockies.  The rivers originating in the NE don't penetrate nearly as well, plus, you will run into winter eventually.

That is why we have so many of our bases surrounding that general area: Ft Benning GA, NAS Pensacola FL, Keesler AFB MS, Ft Jackson, SC Eglin AFB FL, NAS Tampa, Ft Hood TX, Ft Polk LA, Ft Bliss TX, Ft Rucker AL, Ft Stewart GA, McDill AFB FL, Redstone Arsenal, AL etc.

At the next level we should prevent them from getting here in the first place, so, a strong Navy is paramount.  This is especially important for us, as so many of our goods are manufactured overseas, and much more importantly, our primary source of energy, oil, is SHIPPED from overseas.

Even better than stopping their Navies from getting here is stopping them from creating a super Navy in the first place.  Who could do it?  Russia or China perhaps, but, China has a much bigger ocean to cross, and, Russia has very limited access to year round deep water ports.  Europe is too fragmented, despite the EU, and is way too burned out from war anyway.

As odd as it seems the middle east is ultimately the biggest threat.  But not the terrorists.  If the "caliphate" could reemerge and coalesce, combining the old Ottoman and Persian empires with the Indonesian contingent, and then create a Navy, that could spell trouble.  Especially with their acccess to 75% of the worlds oil, 1.5 billion people, and 15,000 miles of warm water coastlines and 10 or more major deep water ports.