Yep that pretty much is the jest of it, though a little more detail would be better.

Alright so the year is 1917, the world has been at war for over three years; Russia is falling, America is joining the war, mutinies are breaking out in France, and a number of battles rage across the world from Italy to Mesopotamia, from East Africa to Macedonia, from Palestine to the Atlantic. This is the turning point of the war and the odds can go into either Entente or Central Powers but to the soldiers in the trenches, pilots in the air, and sailors on the sea it is business as usual. During the chaos of the fighting, six combatants, six soldiers of various peoples and lands from around the world, go missing. They are counted among the many MIA but unbeknownst to their superiors their fate is far from normal.

Meanwhile, in Equestria, the world is far different from that of the human world. Harmony and peace is spread across the land under the rule of the Princess all thanks to the efforts of the Mane Six to spread the Magic of Friendship. But one fateful night the six up and disappear, without a trace. In their place are found six dirty, fearful and violent soldiers. No pony knows where they came from nor where the Mane Six went off too. All six soldiers are brought forth to the Princess to understand what has happened. From first observation, the soldiers all seem to act similar to that of the Mane Six, as far as personalities go. Yet they are very different, twisted and morphed by the different cultures they all come from, of lives from a more harsh world, and haunted by their experiences of the Great War. The Princesses, along with help from close friends and relatives of the Mane Six, try to put together what brings these six complete strangers together and what importance they have in relation to the Mane Six. But this comes as a challenge as all the soldiers want to do is go home and prove very restive to the ponies poking and prodding at their past, all while as news of the six disappearance spreads across the land, dark forces set in motion to take advantage of this opportunity.

This is just something I thought up of while doing some WW1 research. Thoughts? Any ideas of what nationalities (German, Canadian, Indian, French, etc.) and roles (tank men, pilots, artillerymen, infantry, etc.) the Mane Six would be replaced with?

Thanks for reading and have a nice day.

Have AJ replaced by an American

5701306 A thing that made me think of this premise was the huge difference between Equestria and the human world as of Harmony and Chaos. The human world in 1917 is a world of chaos compared to the world of Harmony that is Equestria. I guess a good underlying plot is that the soldiers are the complete opposite of the Mane Six as they were bringers of chaos where they were from as the Mane Six were bringers of Harmony to their world. The old Chaos Vs. Harmony conflict. (Good old Discord up to some old tricks, again)

And while I haven't thought about it much the soldiers that would probably replace the Mane Six would be as follows:
Twilight- German (probably an officer)
Rarity- French
AJ- American (or Canadian, Australian, any of the British Commonwealths and Dominions since the Americans are newcomers in 1917)
RD- Russian (Element of Loyalty and the Russian Revolution, a match made in heaven)
Fluttershy- Italian, Belgian, any of the smaller countries
Pinkie- British ("Pack up your troubles in your old kit back and Smile...Smile...Smile...")

5701264
I think this is a great idea. I think that if written correctly, you will be able to get a lot of good publicity. I wish you luck in your endeavor! I also think the RD should be an ace in the Russian Air Force, if that even existed. I'm not great with WW1 trivia.