In late 2015, a gamer on TMP started a discussion about gaming the war that almost happened between the US and France in Mexico after the ACW. This is a very little known chapter of US history, and since war was (wisely) averted by both sides, there's not much American involvement to write about. I got interested in the idea, however, and the more I dug into it, the more interesting it became. While Napoleon III's attempt to convert Mexico into a French colony is not even mentioned in most American history books, it's a very big deal in Mexican history. In wargaming circles the period is called the French Intervention or (rather flippantly) the Mexican Adventure, and It's the war that generated two scenarios that are perennial favorites in the wargaming world: the Battle of Puebla on 5 May 1862 (celebrated in the US as Cinco de Mayo, but oddly not in Mexico), and the French Foreign Legion's famously suicidal defense of Camerón in 1863. The United States was too busy tearing itself to pieces during its own Civil War to intervene, but as the ACW wound down in 1865, Lincoln (and later, Johnson) stationed whole corps of newly-released Federal army troops and a growing force of gunboats and ironclads in Texas, and began making extremely threatening diplomatic overtures to France. In the event, Napoleon III felt forced to withdraw, but with some scandalous manipulation of the timeline, it's possible to envision a short and extremely colorful war.

In just a few months, this never-war became an obsession, and I launched what I have come to call the Stupidest Project Ever™ (because... seriously? All that work to wargame a war that never should have happened?). By the middle of 2016 I had hundreds of painted Franco-Prussian War French troops to fight my (already large) collection of ACW Union troops, dozens of 1/1200 ironclads and wooden steam-powered warships of all varieties for both sides, and two board games to use as the basis for strategic gaming. I set up a web site to organize my gaming of this... uh... "period"..., wrote most of a campaign system to use as multi-player strategic game for generating land and naval battles (the map and OOBs are still unfinished), and ran a couple test games to find rules for the land battles. A friend even contributed his 15mm French Intervention troops (Mexican Imperial units, Mexican Republican units, Mexican militias of both sides, etc.).

Of course, the aspect of this very imaginary war that interested me the most (and the reason this page is here) was the potential naval conflict between the world's two contenders for second-most-powerful navy (behind Great Britain). Predictably, some in the TMP discussion asserted that USN would dominate the French navy, but when I started to analyze the capabilities of each navy more deeply, I had serious doubts. My reasoning follows.

The French had the second largest fleet of steam-powered ships of the line in the world (after Great Britain). The old wooden broadside-armed battleships were already extremely vulnerable to exploding shell, and even with steam engines untethering them from the wind, they were expensive and unwieldy, but both France and Britain kept dozens in commission right through to the 1870s, and for good reason: a broadside of 40-60 heavy cannons is still a lot of firepower. A fleet containing a dozen or more steam-powered ships of the line would still be a force to reckon with, and could probably defeat or drive off most any fleet the US Navy could raise. The 32 pdr smoothbore is considered old-fashioned and weak by modern naval gamers, but in fact it was still a major naval gun of the ACW. It may not have been able to sink or even hole a monitor, but dozens of them firing a fusillade of shot and shell 2-3 times a minute could still have made a lot of trouble for nearly any ship afloat without iron armor, and at close range might still do some damage even to an ironclad.

Stepping down to the smaller major warship classes, frigates and sloops had grown in size and power with the addition of steam engines, exploding shell, and rifled guns, and contemporary navies (including the US) kept as many in commission as they could afford. These ships were large and roomy, kept plenty of stores on board to stay at sea for weeks at a time, and had enough firepower to fight nearly anything but a ship of the line or a short-ranged armored ram. Steam power was still too unreliable and coal too rare to get ships around the globe and keep station away from bases, so full sailing rig was a serious advantage in mobility for ships on far away foreign postings. These were the ships that did the yeoman's work for the navies of the colonial and trading powers of the world. They had the range to appear off any coast, the firepower to destroy any target, and the endurance to blockade any port or hunt down raiders and merchants in any ocean.

In addition to all that, the French were already centuries into their commitment to the guerre de course, and had the second largest fleet of small, armed oceanic vessels suitable for commerce raiding. The Confederacy had proven during 4 years of civil war that with steam power and shell guns, even a small, rag-tag, piecemeal navy of converted merchants crewed largely by ex-civilians could devastate the trade of a major trading nation like the United States. The French would begin any war with many times that number of vessels capable of commerce raiding, all of them designed and built for war and crewed by professional sailors, so her effectiveness at clearing the sealanes of US commerce would have been many times more effective.

By the end of the ACW, the US Navy was the largest in the world , in sheer number of vessels. However, most of these ships were small gunboats for littoral or riverine duties, and a great number of them were poorly built or just worn out. Nearly all of the 90-day gunboats were sold off or broken up in the late 1860s; so were nearly all of the other types of small wooden riverine and coastal craft.

France was a leader in ironclad development and construction. La Gloire was commissioned in 1859, and by 1865 Napoleon III's Second Empire had 17 similar ocean-going broadside ironclads in commission, and more under construction. Unit for unit these were not as well armored as US monitors, but their armored broadsides were considered invulnerable to most naval ordnance. They were large, roomy, and still had sails, so they were capable of meeting global commitments on far-away foreign stations as well as any frigate, sloop, or corvette. The US Navy had only the New Ironsides in this class, and she was smaller, slower, and weaker than the French and British broadside ironclads. (USS New Ironsides was also destroyed by fire late in 1865, but a wargame of a "what if" war can ignore that inconvenient fact...)

The US fleet of ironclads was the largest in the world in numbers, and its units had more armor than those of any other navy, but this arm of the navy was wholly incapable of operating beyond the American coasts, or even very far from bases. A few large monitors were originally intended for trans-oceanic voyages (Dictator, the Miantonomohs, and 5 unfinished monitors), but actual attempts at such voyages in the later 1860s and 1870s proved just how wet and perilous they were at sea. They would have been useless in anything except the calmest of seas.

Monitors were designed to be hard to kill in two ways:

By contrast, broadside ironclads had huge vertical areas to protect, so carried thinner armor (typically around 4"-5" of iron) backed by 1-2 feet of heavy hardwood (teak, oak, etc.). This armoring scheme proved to be extremely tough during the ACW - most Confederate ironclads were similarly armored - but could eventually be battered open by the huge Dahlgren guns of the Union fleet. The French broadside ironclads did not have the sloped sides of the Confederate ironclads, so would have been damaged sooner under a hail of solid shot weighing 90 to 440 lbs.

The French navy was the first to adopt shell-firing guns at sea, deploying the domestic Paixhans guns starting in the 1840s. Paixhans was a munitions expert and theorist who recognized that exploding warheads were the future of warfare. The biggest problems with shell guns before Paixhans' designs were inaccuracy and unreliable fuses, so he solved these problems by inventing a new type of shell and firing it at a very flat trajectory from a large (8.6") gun. This proved so successful (as at Sinop in 1853) that the ironclad naval warship was invented to counter it.

Paixhans guns could not fire solid shot, and they turned out to be inadequate against ironclads, so the French navy converted to large-bore muzzle-loading rifles (6.4" and 7.6" MLRs until about 1868). Like most rifles, these fired much heavier shot than smoothbores of equivalent caliber, because the ammunition was usually elongated and conical rather than spherical.

Dahlgren had deliberately set out to improve on the Paixhans gun, by designing his guns to use shot or shell at the whim of the gun captain and improving accuracy at range. As the ACW wore on, the Dahlgren guns grew from 8" to 9", then 11", then 15" bores, and the extra size made them very slow to load and too heavy to carry in large numbers. However, the guns proved to be very effective at contemporary naval combat ranges (under 1000 yards), and proved capable of penetrating Confederate armored sides. Since the French ironclads had armor of similar thickness (but much less slope), they too could have been damaged by Dahlgrens. At the end of the ACW, Dahlgren had developed a huge 20" gun, which was meant to go into USS Puritan, but the end of the war negated the need for these, and the four built were never used in combat. In an 1866 naval war with France, they might have been used after all. On paper these guns look formidable, perhaps irresistable with contemporary armor schemes.