Hypothesis: what would the US have to do to defend the Monroe Doctrine if the SS Virginius Affair ha

The answer to the question is that the Americans would have to make do with coastal defenses and build their own version of the HMS Warrior or Black Prince.

What they could build is something indicative here.

Re de' Italia (Built in the United States)

and here.

USS New Ironsides.

It is likely that seagoing monitors like USS Dictator would also be contemplated. These would be repeats of US Civil War designs and would take about 18 months to construct. Considering that a "hot war" would be upon the Grant administration, and the impetus to avenge an inevitable and probable port bombardment would be a further incentive, I think a Spanish American War of 1873 in the Caribbean Sea would be somewhat like the situation between Italy and Austria in the Adriatic with the Spaniards cast in the role of the Italians. The land campaign would be somewhat curious since the American Army would have solid cadres of Civil War veterans. The American navy would be in a similar state, but whether their commerce defense and blockader and port assault experience translates into actual fleet action experience or whether they could find a Tegetthoff of their own... I leave others to speculate.

My opinion is that the Spaniards would find such an escalation and American mobilization... problematic. Spanish Ironclads were dependent on foreign assistance construction and expertise, either British or French. The Americans rolled their own out. Unless the French pitched in with aid, the Spanish will find themselves in this hypothetical situation outclassed eventually. It could take a year for the monitors to show up, but off Key West they will show and then Cuba will fall very quickly. After that happens, it becomes academic whether US ships can sail across the Atlantic to attack Spain. There will be no need. Spain's armored warships without new world ports or coaling supply points will be neutered and will be just as range limited as US monitors. It might become a commerce war a la Kearsage and Alabama at that point, but it seems that Spain would sue for peace rather than have her remaining (Pacific) empire dismembered by US steam and sail frigates of which there were more than plenty to cover those Pacific possessions and take them.

USS Dictator took about a year to build. 

USS New Ironsides took about nine months.

USS Puritan, a "Theseus ship" was laid down during the Virginius crisis and was 80% complete when laid up in ordinary after the crisis was resolved. That was seven months. If the war had gone hot, I estimate another five months. The actual Spanish American War of 1898 lasted about eight months.   

As we get into the New Steel Navy of the 1890s the build times take four to six years, however the USS Kearsarge laid down during that Spanish American War of 1898, took only two years.  War speeds things up.

The US bought those ships to keep the Spaniards from buying them. They did not need them. As an aside, the Americans, after they used the two ships came to some interesting conclusions about British built warships.

1. The engines were better than their own. The Americans copied features in British powerplants that they liked.
2. Crew accommodations and ship work spaces were horrible. The Americans despised these features and considered British practices and accessions unsafe and intolerable. 
3. British rapid-fire guns were faster cycling than the equivalent bore American artillery, but the guns were unsafe with breech plug jams and blow-outs common and barrel bursts from faulty British supplied powder propellant charges being all too common. The Americans landed the Elswick artillery and switched to American ordnance as soon as they could.
4. Bearing 3 in mind, and 2, the Americans found British ammunition and propellant stowage systems unacceptable and hazardous in the extreme. Kind of ironic when one remembers what happened to the USS Maine, which used those methods originally?
  
 

 

   Guys

All good points. However Valparaiso happened in 1866. The French were in Mexico about the same time, the Americans were fresh from their own Civil War and six years later is not that long to make trouble, no matter the Franco Prussian War or The War of the Pacific or the never-ending Carlist-Isabelline fracas that led to the Franco Prussian War. Example...

France was in not too good a shape after 1874. Ten years later the French navy was on a rampage in the South China Sea and this upset a lot of nations; Vietnam, Great Britain, China, Japan, Spain, the United States etc. 

Just because the homeland is in bad shape does not prevent the people out in the colonies from screwing up and dumping a major war in your lap. The SS Virginius affair or the USS Maine Incident are other examples of things going to 6s and 9s in a real hurry.  The Spanish republic handled the SS Virginius affair sort of, but the Spanish monarchy of 1898?  They mismanaged a worse political situation at home than they had in 1874 and earned a war on top of it.  
  
All it would take is another incompetent like Sigismund Bermejo or a Weyler and an eager firebrand like Teddy and away we go. 

The Panic of 1873.


The US banking system was fluidic  but not backed by sufficient hard currency reserves on hand to meet expected loan obligations and demand calls. Railroad bond issues of this panic resemble the junk bonds of the tech bubble during the Clinton administration with similar disastrous future effects. 

However, we can add the Franco Prussian War, the demonetization of silver as a reserve hard currency in the US and Germany, runaway American inflation, a farming crisis and British banking manipulation that backfired when the US railroad bonds that the British lending houses used as "capital" proved to be worthless as sureties.  

How would the Grant administration possibly finance a naval war and a land campaign in the face of that economic malaise? If Salmon Chase's Civil War example was any guide, the Americans would float bonds, impose import duties on foreign imports, place large armament orders in excess of immediate need to create an artificial demand economy, and go into deficit spending by increasing the public debt under the cover of fighting the war and actually devaluing the currency. They would be running the printing presses and in effect screw the European creditors over by devaluing the dollar and paying off the aforesaid Britons and Germans in "greenbacks".

It might make sense to build a navy and keep it in permanent ordinary as is done today to maintain a steel industry. 

Foreign capital is probably not going to be available anyway, so it will be on the backs of the American working poor that the internal capital and financing will be raised. A wartime government can find a way to superheat economic activity and generate (false) revenue streams. (WWI and WWII). 

Who knows? If Grant's cabinet full of crooks play the angles, they might even win the war before the bottom falls out of the American economy. Taking Cuba would be an $800,000,000 windfall. Just how much of the global sugar market did Cuba supply?

As for the American war machine, that is somewhat problematic. British and French steam engine technology was about a half decade ahead of American tech. Artillery and firearms; the Europeans were similarly about a full decade ahead. However... war brings innovation and I presume parity. The Spanish are ratio-wise economically outmatched 4 to 1 in capital, 8 to 1 in industry, 3 to 1 in shipyard capacity, and potentially 5 to 1 in manpower. Add to this problem that US relations with France are somewhat friendly and that while the British would sell tech to anybody except the United States, there is GERMANY whose tech (Krupp guns are being sold to anybody with the cash including South American republics, China, Thailand, Russia and Turkey, so why not the Americans in this circumstance?) is right up America's alley. The Germans are trying to break into foreign markets. Those are German steel rails on American sleepers, so why not German steel breech loading guns or licensed copy designs in American ships? That would be better than anything the British or the French could offer. 

The Americans could make do with their existent repeaters for this exercise in  folderoy war-making until Lee comes out with his vertical stack bolt action. 

Mister Gatling is already putting out better Gatling guns.  

I wonder if the Spanish can or would get similar help?  Very likely. France would be more than willing.