We all know how, while the US was distracted by the American Civil War, France tried to install the unfortunate Maximilian as Emperor of Mexico. That didn't work out so well; Max ran into various troubles, and the Union victory sealed his fate.
Less well known: there was a second European violation of the Monroe Doctrine during the Civil War.
"On March 17, 1861, Santana announced the annexation of the Dominican Republic by Spain. A number of conditions had combined to bring about this reversion to colonialism. The Civil War in the United States had lessened the Spanish fear of retaliation from the north. In Spain itself, the ruling Liberal Union of General Leopoldo O'Donnell had been advocating renewed imperial expansion. And in the Dominican Republic, both the ruler and a portion of the ruled were sufficiently concerned about the possibility either of a renewed attack from Haiti or of domestic economic collapse to find the prospect of annexation attractive.
"Support for annexation did not run as deep as Santana and his clique had represented to the Spanish, however. The first rebellion against Spanish rule broke out in May 1861...
"Resentment and rebellion continued, fed by racial tension, excessive taxation, the failure to stabilize the currency, the uncompensated requisition of supplies by the Spanish army, heavyhanded reform of local religious customs by an inflexible Spanish archbishop, and the restriction of trade to the benefit of the Spanish empire. The Spaniards quelled more uprisings in 1863, but guerrilla actions continued...
"Circumstances began to favor a Spanish withdrawal. The conclusion of its Civil War promised that the United States would make new efforts to enforce the Monroe Doctrine, which barred European powers from the Western Hemisphere. Spanish military forces, unable to contain the spread of the insurrection, lost even greater numbers of troops to disease than they did to the guerrillas. The O'Donnell government had fallen, taking with it any dreams of a renewed Spanish empire. On March 3, 1865, the Queen of Spain approved a decree repealing the annexation of Santo Domingo."
http://www.onwar.com/aced/nation/day/dominican/fdominican1861.htm
So: WI Spain had tried to hang on to it?
It's a stretch. The DR had low value, Spain was broke, and it brought the Spanish Crown into not one but three possible conflicts -- with Haiti, with Dominican rebels, and with the US. OTL, quietly backing away was the course of wisdom. But governments aren't always wise, so [handwave] let's say Madrid tries to keep the DR.
First question: how does the US (and Britain) respond? -- I suspect they'll let it pass. Okay, it's a formal violation of the Monroe Doctrine. But OTOH the Dominicans have /asked/ to be recolonized. It's pretty clear this is an unusual situation, unlikely to be repeated, and I don't see the Johnson administration as spoiling for a fight.
Next question: can Spain hang on? Well... I think this depends on where we put the POD. If it's "Spain acts as in OTL, except that it tries to keep the DR", then I think not -- the Spanish had made themselves very unpopular, and I don't think 1860s Spain was in a position to fight and win an open-ended colonial war. In this version, Spain hangs on for another year or two, then gets kicked out.
A more interesting variant is, Spain is less obnoxious -- less blatantly racist, doesn't send an Archbishop who starts kicking over local customs, etc. etc. I don't think Spanish colonial rule could be so benign that there'd be /no/ rebels, but I think it could be good enough that there'd be a lot fewer than iOTL. Remember, the DR was poor, isolated, and under constant threat of reconquest by Haiti. If Spain rules even not-so-badly, I think they have a shot at hanging on.
Then what?
I think a recolonized DR is going to have significant long-term impacts on the Caribbean and Latin America generally, and maybe on Spain as well. I have some thoughts, but I'd like to hear what others think. Anyone?
Doug M.