What it says on the tin. The ideas behind the Ostend Manifesto lead to a war between Spain and America over Cuba. For a specific POD, lets put it as the Black Warrior affair. Could it be called the "Black Warrior War"? Who wins?
At this time the Spanish navy still had a decent power projection capability. In the Chincha Islands War (1864-1866), they sent a very good ironclad and six very powerful steam screw frigates. Isabella's Spain also invaded and fought against Haiti and rebels in the Dominican Republic 1861-1865.
You are dealing with a country with the world's 4th navy during this era - relatively, they are much, much stronger compared to the US than they were 1898.
And describing the US army of the time as a great force - in either quality or quantity - is exaggerated, to put it mildly.
It was enough to put down a rebellion, but not enough to challenge a washed-up former great power.
The US Army of 1865 is an entirely different beast than the one of 1855.
Yeah, an 1855 war would be an even affair - but Spain got bases near the US, the US does not have bases near Spain. The Spanish have 15 steam-powered long-range vessels. How many do the US have 1855?
Like all wars the US has fought, victory depends on rallying the public to support the war. If Spain is intelligent enough to limit the fight to the Caribbean and Gulf than it is likely that support in the North will be limited. But I view that as unlikely. First there will be, given the long period of animosity between the US and Spain over the territorial expansion of the former, the urge on the part of Spain to "teach the US a lesson. This means bombarding a port city or two. If any city North of Washington is struck than all bets are off. The Northern politicians don't want to be seen in the same treasonous light that killed the Federalists after the War of 1812. So it is likely that they will, grudgingly, support the war effort.
With Navies being nearly equal than a long term conflict will probably arise, and unlike a war against Britain this conflict gives the US the upper hand. US industry will very quickly outpace that of Spain's and we'll see earlier use of ironclads and repeaters.
Politically, the outcome will be interesting. The major powers of Europe are involved in the Crimean War and so will not want to get involved in this affair. In the US the surviving Whigs, northern Democrats and new Republicans will almost certainly come together to attach a new Wilmot Proviso to the bill that funds this war. This time it will almost certainly pass and thus guarantee that any territory acquired from Spain shall be free territory. This will infuriate the South and Pierce will probably veto the measure which in itself will lead to chaos in Congress and delay funding for the conflict. In the end its likely Douglas will lead the way for a compromise that makes the Nebraska Territory free in return for eliminating the new Wilmot Proviso.
Some possible knock on effects.
Pierce isn't dumped by the Dems and wins reelection in 1856 in a close sectional race.
Douglas angers his Northern supporters enough that he losses the 1858 Senate seat to Lincoln.
Kansas comes in as a Free State as a sop to ensure war funding.
US Navy expands a lot and a large number of future Civil War leaders get valuable on the job training and a few of them end up dead.
Dems lose even more support in the North, especially after Dred Scot goes back on the War funding agreement and essentially reopens the remaining Nebraska Territory to slavery.
Civil War occurs on schedule but US better prepared for war and with a larger navy that makes Britain a bit more nervous.
Benjamin
benjamin, it's very doubtful the expansion of the USN will reach a point that gets the British nervous. To defeat Spain I wouldn't be surprised if the USN doesn't even rank in the world's top five fleets at the end of the war. The British may even laugh as this will likely leave the US with a larger and more expensive wooden fleet just as ironclads become the smart investment.
The question is going to be the army. Enlisted men went overwhelmingly for the Union OTL so if the North has tens of thousands more trained regulars, some of them even combat veterans, at the start of the war...and since there was a marked preference for Southern officers in the army pre-ACW it seems quite possible that some of those officers do not live to see this TL's ACW.