You could be right, but not for the reasons you suggest, and I think you do not speak for most Brits. In fact, most Brits want their boys out of Afghanistan and wish they'd never gone in there at all ... We of all nations must know how futile it is to try to subdue the Afghans.
But we are there, along with America and our European allies: France, Poland, Spain, Norway, Sweden, Lithuania, Finland, Sweden, Iceland, Austria,
Netherlands, Romania, Denmark, Czech Republic, Hungary, Macedonia, Albania, Georgia, Portugal, Ukraine, Ireland, Bosnia and Herzegovina, Germany, Italy, Turkey, Bulgaria, Belgium, Croatia, Slovakia, Azerbaijan, Latvia, Estonia, Greece, and Luxembourg. As well as, of couse, many non-European nations.
We have more people out there than they do? So what? All that proves is that we are more belligerent than they are - something that never needed proving. But isn't it good to see how our rebellious offspring follows in our footsteps so closely?
Constantinople? Very good, den! Although it fell before Rome did, and I believe the adherents of the Western Church sacked that city long before the Turks got there. I'm not sure how the parallels run here, but I'd hate either Europe or USA to turn against the other in the pretence of rescuing it.
As far as I can see, America's imperialism is represented by a commercial empire. Politically, the USA seems to be introverted, xenophobic and wholly self-interested. The contrast is irreconcilable! Politically it has few "overseas territories" (Newspeak for "colonies"), but its business interests reach into every country in the world.
Inevitably, USA will diminish and fall, just like every other imperial power before it, and new ones - China, India, Russia (yes, you read that right) and Brazil, will take over ... for a while. Meanwhile, we would all be grateful for as much benign wisdom as the US is able to show the world, starting with Copenhagen.