(I haven't read Rome Sweet Rome. This is me pulling things from my ass)
This assumes I'm in charge (Col. Peterson, 35th Marine Expeditionary Unit, thank you), that the MEU travels from Afghanistan to within a few day's march of Rome, and it lands in 23 B.C.
By that year, Augustus had seriously drawn down the army in a massively short-term expense, leaving a lot of Italian farms to veterans and going from 60 legions down to 28. Legions were chronically undermanned to begin with, so I'd be facing about 135,000 men, not 330,000 as popular mechanics mentioned.
So let's say my entire MEU is on the same FOB (bullshit, but whatever). That gives me about 2,300 Marines and sailors, plus a handful of contractors or so. We go to bed in Helmand Province and wake up in Italy the next morning with a legion (A single one with engineers and cavalry, maybe 5,000 men) outside the perimeter. GPS, satellite and comm outside our own nets are down, but we have power.
The Romans attack first. Counterattacks are proportionate. Arrows and pilums will be met with small arms and machine gun fire. Trebuchet volleys will be answered with 155mm howitzers (I've got an artillery battery with me, and while GPS is down, there's still polar and shift, and lensatic compasses work just fine).
That first encounter WILL be a rout of Rome. Send my QRF out and see if there's any wounded I can have treated and interrogated. Launch a raven (drone) to recon our area and have my S-2 start making new maps.
By dinnertime I'll confirm the approximate year and location I've been transported to, and start weighing options. They soon boil down to "conquer quickly or be wiped out slowly."
So we prepare to sack Rome. First things, preparations.
- All personnel ordered to have a bayonet or Kbar on them at all times. Pre-gunpowder times means we keep our pre-gunpowder weapons close.
- Scour my ranks for anyone with a schoolboy's knowledge of Latin or greater. Get them to work making phrasebooks for S-2. Language barrier is gonna be big.
- Scour my ranks for anyone who can competently ride a horse. Commander's intent is at least a company-size cavalry element.
- Engineers retrieve any siege engines from the battlefield and see how we can reverse-engineer and/or improve them. Steal bodies from the avionics and airframe platoons of the air element and whoever in CLB you need.
- MCMAP instructors start bringing out the knife and bayonet sections of the course and begin instructing the entire MEU up to brown belt level. Just on those portions.
- Shut down my armor, AAV, and LAR platoons. They guzzle fuel, the armor is excessive for our needs and there's little we can do with the 120 we can't do elsewhere. Strip them down for parts and easily cut sections. Cycle the personnel into the infantry (and cavalry, for those skilled as such).
- EOD, we now have a lot of tank rounds we won't be using. See if we can rig these with some sort of proximity fuse so we can launch them by trebuchet and magonel. Same thing for 155. Howitzers might wind up being too heavy for us to take, but we can use the ammo at least.
- Infantry start foraging parties. I have 2300 mouths expecting three squares daily. When possible, buy all the food they can. With the junked tanks, AAV's and other vehicles, we have a trade good in high-quality steel scrap, which any local blacksmith will shit his pants over. Speaking of blacksmithing, see if we can recruit a few. Our O/A welding rigs will run out of fuel quick, and my engineers will need some education in the old-fashioned methods.
How I actually take the city will depend on reaction. Ideally I'd keep any bird bigger than a raven grounded until I see troops massing, then send my cobras on strafing runs. Gatling guns and hellfire missiles are gonna be hell on a legion camp. But I'd likely need a ground/air assault to get as much of my people and stuff within the walls of Rome period, and set up my new OP there.
So, that's my from-my-ass before I've had coffee on a lazy Sunday morning response.