As a resident of "this part wouldn't be that tough", i disagree. The entire Cascade range extending from the top of the central valley to the top of Washington, extending out to the ocean, is very very rough, steep, heavily forested terrain that would be absolute hell to get any kind of equipment through if the locals were to blow up a grand total of three bridges on three separate highways.
The USA is particularly strong from that side. Most of its navy patrols those waters, many islands on the Pacific are USA territories all the way to Japan which itself acts as an arm of the USA Military since the forced demilitarization of Japan in WWII. Japan and other islands basically form a wall around the eastern mainland that would be difficult to cross undetected, and when they did reach the shore they might get flanked from Hawaii, Panama, or Alaska.
There is a reason Stalin chose to ally with and provide nuclear weapons to Cuba, and approach the USA via the gulf of mexico and Atlantic.
Nope, logistics is the first enemy. You can have the men and high tech weapons you want. But if you can't get them to where those things are needed in the quantity they are needed-- You lose.
The US military is still the best in the world at moving men and material in the least amount of time than anyone else. And as much as fuckcars hates the over grown US highway system, it would be a tremendous advantage to be used against any would be invader.
Also: the emerald triangle and that whole area of NorCal are the far-right wackos who regularly fly the state of Jackson flags, they're heavily armed and waiting to shoot people up there