Also, the oilfields in Texas/Gulf, which are almost empty by now are suddenly full to the brim, again. While the US loses foreign imports, internal production has suddenly risen by a vast percentage. In the Appalachians, huge, empty mines are suddenly full again, whole mountains are back in place. In fact all the mines on the whole continent are full again. You simply have to go to the archives and actually know where the stuff was, with actual maps of the underground where it had been. Refineries and everything already exists, you only have to make a (relatively quick) dig down to get it. Vast areas of Canadian soil are free to settle, and the Great Plains are fully fertile again, with modern agricultural tools available.(Which means we can get by without vast amounts of fertilizer for a year or two). And people know of catastrophes that happened and how to avoid them.
While there will be rationing, the sheer amount of new jobs available will help public to accept the changes really quick. With no foreign goods, everything has to be restarted internally. Yes, the diet will be mostly corn at the beginning, but every kind of production will suddenly be needed. Mining will have an upswing, construction, everything. General motors and Ford may have a downtime, but only to retool. Merchant ships would be sent out with gold to procure raw materials need. A war economy would be initiated immediately in order to consolidate and rebuild.
And there will be war. England, France and Spain just lost vast territories to an unknown foe. Expeditionary troops will dispatch almost immediately, especially from Britain and Spain.
But these conflicts are negligible, simply the fact that contemporary units were used in huge formations and completely dependent on officers holding the troops together and coordinating them is deadly against a foe with the ability to kill at the boundaries of human visual recognition. Even modern hunting rifles are sufficient to eliminate the whole officer corps of an army before you are found. Especially since these officers are easy to identify, even if they suddenly were to use the same uniforms as standard soldiers. They won't learn fast enough. A handful of machine-gun and mortar squads will simply mop up the rest. You won't even need tanks or aircraft, except for making a point.
Also, naval supremacy is a given, the fastest warships these days rarely made more than 12 knots (in one direction only), and ranges were well below a mile, with accuracy only somewhat present below a couple hundred yards. And highly flammable and easy fractured hulls. A single mortar (especially with white phosphorus) would enable a Coast Guard Cutter to go mano-a-mano with the best of the Royal Navy. A freighter with a single 155 and a couple of machine guns would terrorize the seas, and need no further escort but troops to secure it at harbour...
Imperialism is not only a possibility, it is a given thing. I fully expect the United States of America to extend to both Americas (which at this time still were connected, no Channel) within a decade. It is almost a necessity in order to consolidate the new/old lands.
Another thing in favour is that the US would not have to fear any initial espionage, as no 1770-er could hide in our culture for long without outing himself.
I also honestly doubt many people would leave the US (With what? No airports anywhere and most blue water ships gone.) to educate the world. After all, while being rationed in the US for a few years would be kind of bad, outside live would be worse. In the US, you'd have a steadily improving status with at least basic infrastructure (and probably cinemas and other amenities on a time-table basis), medicine and other stuff, while outside, you would have absolutely nothing, and would have to start building everything from scratch. There will be some people that manage to get out, but they'd need years of tutelage to even start having any impact on stuff.
Even if they took an engineer's handbook and all the blueprints to our stuff with them, there is so much interconnecting tech needed.
For instance, at building modern furnaces in order to make the materials needed for modern tech:
You would need a small, very primitive one to start building the tooling to build a better furnace, to build the piping needed for a decent one, to be able to build the stuff needed to drill for gas to fire the burners for the turbines to generate electricity for the modern furnace. Oops - you need a turbine, well, for this you need....
Ok, let's use water power instead of gas, or wind? Same problems, and btw., these guys don't know about electricity, yet, so you have to teach them all this, as well - cables, transformers, switches, etc...
Oh wait, you still need plastic to isolate the cables, cranes to move the huge coils carefully...
And now remember that you are the only one knowing about this, but have never, ever made that stuff by yourself, by hand. And all the 'students' are just watching you and scratching their heads while you try telling them what to do.
Most people would simply give up after a while, without much progress.
The Greeks are somewhat undependable allies when it comes to keeping promises. I am sure the fleet of 300 galleys they promised will turn out to be 3 guys in an oversized cooking pot. (Thanas, revealing the plans for German world domination)I do archery skeet. With a Trebuchet.