He didn't need to imagine it, he observed Nazi Germany do it.
In 1984 the armies of Oceania are roaming the world fighting war after war against an enemy today who was an ally yesterday, with the population at home blissfully unaware and unconcerned, and that state of affairs is maintained by constant propaganda and the constant rewriting of history so that the record shows that today's enemies are villains of long standing and today's allies have always been allies.
But, if you think critically, there is a flaw in Orwell's thinking. I just finished 'The French Revolution and Napoleonic Wars' by Great Courses (incredibly good). Napoleon's armies fought constant wars for 10 years against most of the world, and even after being defeated in Russia and losing hundreds of thousands of men, the French people never demurred. Napoleon was exiled and fought his way back, to the acclamation of the French people. Which reminds me of this quote ... from the oldest thing written, 2500 BC, on stone tablets,
"He has worn us down like a river flowing through a gorge. Like a waterfall splashing on the rocks. So much life, so much energy. He makes perpetual war you know? He spends our sons like so many arrows in his quiver. And for what? For the excitement of a battle charge. He if forever building new towers. For what? For the sake of touching the sky. You see the wall that surrounds Uruk? Who do you think toiled under the sun to drag the stones? We call on the gods to spare us, but on and on runs Gilagmesh, with us for the soles of his sandals."
So, nothing had changed from 2500 b.c. to 1890 a.d.
Now I've just started Buchanan's 'The Unnecessary War' which begins with the machinations of the European states, Britain, France, Germany, Russia and China and Japan from 1900 on ... and there is a constant shifting of alliances, and wars fought, leading up to WW I, and for all practical purposes the populations of the countries were unaware and unconcerned. Then, there is this incredible quote (Malynski) from 1929 ...
‘It is only when we realize that France was invaded by hundreds of thousands of inhabitants from Massachusetts, Pennsylvania, Florida, Illinois, Wyoming, California, Louisiana, and subsequently from Ontario, Manitoba, Rhodesia and New South Wales, whose only possible motive was to hasten the triumph of democracy, that we begin to understand something of the power of Israel. The power to stir up a whole nation of solid, egotistical and utilitarian individuals, and to persuade them that their greatest privilege is to set out and get themselves killed at the uttermost ends of the earth, with no hope of gain for themselves or their children and almost without their understanding against or for whom they are fighting, or why, is a simply incredible phenomenon which makes one afraid when one comes to think about it.’
The point is that governments have always been able to get their populations to actively and even enthusiastically support wars and the slaughter of the people of other nations. All the mind control mechanisms described in 1984 are gratuitous, superfluous.
So, the fact that the US population is unaware and unconcerned as the US armies decimate the middle east is ..... exactly what one would expect from examining 4000 years of human history.
