Early 20th century
Issue: Home Rule for Ireland
Location: Ireland and Great Britain
Pro: (political) Many Irish Catholics, British Government, Irish Americans
Pro: (Combatants) Irish Republican Brotherhood, Irish Citizen's Army
Cons
political) British Army, Ulster Protestants
Cons: (Combatants) Ulster militant groups
When Home Rule was voted in, and the Irish Protestants vowed to resist with force, Officers that did not live in Ulster were not given a choice to opt out of possible opperations to supress the Ulster resistance. Some leave the service, and the army is effectively paralyzed and unable to enforce the will of Parliament. The navy is unaffected--and will remain so as long as it's not directed to intervene.
The Orangemen refuse to acknowlege Home Rule, and resist with force. The various Irish independence movements, acting in support of a British law for a change, fight--after being declared to have police powers by the home rule government (which is primarily southern due to the refusal of the north to participate in the new government.)
Ulster sympathisers see that the Ulster forces get military support, while the British govrnment is forced to support the Irish parliament with supplies.
Making matters more complex, volenteers from America join the fight against Ulster.
In the middle of all this, Archduke Franz Ferdinand is assassinated, and matters spiral out of control in Europe. Britain is more cautious about entry into the war, with a mess in Ireland.
Now the British government needs to try to bring about peace, and fast.
In the end, I suspect that a partition is worked out, but some of the 6 counties of modern Northern Ireland are part of the Irish Free State.
RL notes: there were armed groups in Ulster pledging to fight to defeat Home Rule no matter the cost, and they were much better armed than the rebels in Easter, 1916.
There was also serious doubt about the willingness of the British Army to fight if Ulster did fight.
With research, this might be worth developing--any thoughts?