FWIW,this is the video that inspired this thread (well, that and I’m reading Ring of Fire again). It’s a YouTube video by the guys at Military History Visualized which is a site I’ve been following. I had seen this video in April but watched it again yesterday and was thinking about the what if scenario I laid out in the OP. While I was thinking of it I thought about the modern vehicles and thought that would too unbalance things, as even an armored calvary force could probably go through the French army like a hot knife through butter. Even if you just allowed jeeps or other logistics vehicles I think that it would unbalance the situation too much to make interesting. I considered giving the modern soldiers a few thousands of the BEF but then not sure about logistics support for the period troops…or how they would handle integration and communications on such short notice (i.e. arriving the day before the battle historically took place). I’m also not sure about French scouts and whether or in what strength the French were in the Waterloo area prior to the battle. My knowledge of the actual battle is mostly from fictional account from stuff like Sharpes Rifles and the like, though I had a course (over 2 decades ago) on European military history that covered the battle at one point.
Also, does your average infantry company have any significant number of claymore mines or other stuff like that?
Seems to me that you could very easily wreak terrible carnage among Napoleonic era troops with such things.
Smallpox is a virus and there is no known cure. Once contracted, the victim is in a world of hurt.
Put the cannon on the back side of a berm or even minor fortification and they become invulnerable to gunfire.
Since this is pre-Geneva convention you could just gas the hell out of them. So… gas masks, hazmat suits, and about 10,000 WW1 gas shells of varying vintage, and let fly.
Note: I know nothing of warfare or the conditions of Waterloo so have no idea if gas would work. But it would demoralize the shit out of the survivors.
Pretty sure that artillery of Napoleon’s day didn’t typoically do indirect fire, if only because there were few good ways to communicate and correct their fire without radio.
They did have howitzers (and mortars of their own), of a sort, for indirect fire (at least the British did…I’m fairly sure the French did too, but I base that completely on Empire Total War, so I could be wrong ;)), but yeah, most of the cannon of the period were direct fire weapons. Wouldn’t matter, since putting their cannon on the reverse slow wouldn’t help them much wrt mortars which could easily outrange any of the cannon of the period and would slaughter the crews.
You push them to the top of the berm to fire.
And then the crew dies.
Not necessarily, you just push until the mouth of the cannon has LOS to the target, and fire from behind.
The M-203 has a max range of something like 400 meters (430 odd yards)…that puts it well within the range of most Napoleonic era cannon if they actually want to hit something. I’m trying to remember, but I think the effective range of cannon in that era were something like a thousand feet or less if you wanted it to hit close to what you were aiming at, though obviously you could shoot it from further out (half a mile maybe?). Moving it up into firing position from the slope would lower the effective range even more…plus, I don’t know about you, but I wouldn’t want to fire said cannon while standing behind it on a slope. 
Neither do I, but then I would rather not be less than 100 kilometers from any battlefield ever.
How do you aim?
With great difficulty.
I wouldn’t make that assumption. Not that modern soldiers aren’t better trained in a general sense and probably healthier on average ( at least in the short term, depends on how long this “war” goes ), but 18th-19th century European armies lived on parade ground maneuvers and heavy drill. Napoleon in particular was obsessed with speed and his troops were used to rapid movement ( in their contemporary context ) and they were marching 25 miles a day in Russia, a country where he needed more cumbersome logistics than in France.
Modern armies simply don’t have that kind of continuous experience humping it every single day without motorized transport. This is one area where I wouldn’t give the modern armies any particular advantage - at best it would be a wash, at worst modern troops might be at a slight disadvantage. Never underestimate an opponent ;).
True, and good point. I was thinking of modern diet and training trumping poor training and poor diet, but ‘poor training’ had more to do with the fact that a lot of armies of that period didn’t train in how to do aimed fire (as opposed to rate of unaimed fire). I do think that the French logistics tail would slow them down substantially, even if you are right and it’s a wash wrt marching speed/distance of individual soldiers. It’s simply more difficult to march and support 70k+ troops over long marches in hostile territory than a few hundred, and the modern army supplies would be a lot lighter and easier to move more quickly…and, presumably the modern troops wouldn’t necessarily be in hostile territory, though gods know what the locals would think of them. Their training and disposition would most likely keep atrocities against the locals down far lower than any early 19th-century army of the day, however, especially the French during that period (IIRC it was French foraging practices that soured their relations with Spain and several other countries, though I might be mis-recalling).
If I was the commander of the infantry company, I’d get a handful of the designated marksmen(not quite snipers) and detail them to pop artillery crews, while the rest would be detailed to hit anyone on horseback with the infantry and/or anyone wearing a gorget or epaulettes.
Then I’d let the riflemen, mortars and machine gun crews deal with the regular infantry.
You know, we’re all wrong. 
You only need one AMERICAN soldier. After all, they’re an ARMY OF ONE and the French are cheese-eating surrender-monkeys, right? 
An SF team would probably have half of Napoleon’s general’s fighting for them before the battle started.
Besides Big NappyB, how many other top officers would need to be killed before the French would surrender? People upthread have mentioned sending back modern snipers, but how many targets would they need to actually take out?