The idea that attacks on the US come from afar is an assumption too.

It doesn’t seem impossible that some radical group of attacks the US from within.

And that’s quite apart from the US threats to attack its neighbours.

I did some digging, reaction engines are jet engines that can transition into a rocket propelled mode. It could enable a single stage to orbit plane.

Within the atmosphere the jet engine will burn atmospheric oxygen which would theoretically reduce the amount of fuel needed to reach a certain delta-V. Once outside of the atmosphere the engine switches to a rocket propelled mode, usually by injecting stored oxygen into the engine in addition to jet fuel.

So far there hasn't been a working prototype of a reaction engine. A British company SABRE closed down due to lack of funding but it did prove several pieces of the reactive engine design could work independently.

North Korea and Iran both have orbital launch vehicles. If you can put something in orbit, that's most of the way to putting something in Times Square or DC.

This hasn't happened yet. And much easier, more deniable attacks like car bombings of these places also hasn't happened to any real degree.

The mass shootings in the US are mostly performed by Americans, usually right-wing Americans, and the percentage performed by foreigners is close to zero. Looking forward to some right wing American replying linking to individual examples of foreigners doing mass shootings as though that disproves the point.