Read “Grand Strategy of the Byzantine Empire”, it parallels with current US situation. When the West Roman Empire fell, the surviving Eastern Empire was stuck with less manpower, financially strap and unable to fight sustain wars. US today is in similar straits. Saddled with debt and unable to afford long costly wars. But like the East Romnans, the enemies do not go away. Islam is hellbent on destroying the US. After Afghanistan and Iraq the US population cannot politically stomach another Bush style Gulf War/nation building. The only thing left is cause the Muslims to fight amongst themselves. That is what the East Romans did. War emphasize usage of good intel to be highly aware of enemy intentions and actions, usage of dirty tricks (Spec Ops and CIA) to cause fights amongst the various enemies and factions within. The military is used for quick and decisive strikes because long campaigns can no longer be afforded financially. That is how the East Romans survive another 700 years after the fall of Western Rome. The US may be in the same situation due to our financial debts.
And then the Byzantine “Holy Roman” Empire got steamrolled by the Muslim hordes who control it to this day; Constantinople became Istanbul.
Even the enemy doesn’t stop learning.