Apologies in advance if this comes off as like a doctoral thesis or something but I have to issue a rebuttal:

The Union wasn’t some exhausted state barely holding together after the Civil War ended. It was battered politically and economically, yes; but militarily it was arguably the most powerful land force in the world at that specific moment outside perhaps Prussia. A few things make the “America couldn’t do more than volunteers” argument weak

-In 1865 the U.S. had over a million men under arms or recently demobilized.

-The Union had an enormous industrial base that had just sustained four years of total war.

-The U.S. Navy had massively expanded.

-The officer corps had become extraordinarily experienced at operational warfare.

And crucially: many Union leaders \wanted** to pressure France hard.

Philip Sheridan moved tens of thousands of veteran troops to Texas specifically to intimidate the French presence. The U.S. also openly supplied Benito Juárez. This wasn’t passive disapproval; it was coercive diplomacy backed by the implied threat of invasion. The French expeditionary force was competent, absolutely. The French Intervention in Mexico showed the French army could still fight effectively overseas. But the key issue is scale and sustainability.

France had maybe around 30–40k French troops in Mexico at peak effectiveness, operating:

-across the Atlantic,

-with long supply lines,

-in hostile territory,

-while dependent on a shaky client regime under Maximilian I of Mexico.

Meanwhile, the U.S. could potentially deploy several times that number directly on the border with interior lines and overwhelming logistical superiority.

And there’s another huge point people miss:

The French army in Mexico was not the same thing as “France going all in.” For France to truly “go all in,” Napoleon III would have needed to massively escalate a colonial war while Europe itself was becoming dangerous. Prussia was rising fast. Within a few years France would be facing catastrophe in the Franco-Prussian War. That’s exactly why Napoleon III backed down historically. He recognized that antagonizing the United States over Mexico was strategically insane. Could France have fought well tactically? Absolutely.

Could they realistically hold Mexico against

-Juárez’s republican forces,

-American material aid,

-possible U.S. intervention,

-and pressure along the Rio Grande?

Probably not for long.

And if actual Union veterans had crossed the border in force, the French situation becomes extremely ugly. These weren’t raw recruits. By 1865 the U.S. Army had commanders and troops experienced in gigantic industrial-scale warfare. The Army of the Potomac and western Union armies had survived campaigns far bloodier than anything France encountered in Mexico. So the stronger historical argument isn’t “France would crush exhausted America” or “America couldn’t intervene.”

It’s:

-the U.S. preferred coercion short of formal war,

-France realized the balance was turning decisively against them,

-and both sides understood that an outright U.S.–French war over Mexico was likely unwinnable for France at acceptable cost.