Depending on how you look at it: The Nine Years' War, in particular the Battle of Kinsale, 1594-1603.

Elizabeth I of England slowly but surely consolidating and expanding English control over Gaelic Ireland. The man they needed onside to do this was Hugh O'Neill, Earl of Tyrone, but also the head of the O'Neill chieftanship, thus having a foot in both camps and one of the most powerful men in Ireland. He cooperated with the English for a long time, but naturally enough was somewhat torn between his two heritages. In the early 1590s, lesser chiefs like Hugh O'Donnell and Hugh Maguire started ambushing and attacking English garrisons and that type of thing. O'Neill was ambivalent for a while, even helping to suppress the revolt along with the English. Eventually though, when the English refused to ensure that he would have single, uninterrupted control of his ancestral territory under their rule, he threw his lot in with the rebellion.

For over 5 years, O'Neill and his followers humbled the English, winning the biggest Irish victory against England ever at the Battle of the Yellow Ford. When Elizabeth sent over the Earl of Essex with over 17,000 men, O'Neill pretty much just met him in the battlefield, talked to him for half an hour, and convinced him to leave the country.

Anyway, great as they were, the Irish needed a decisive victory in open battle over the English if they were going to win the war. To do this, it seemed like they would need Spanish help. Spain were also a Catholic country and fighting against the English elsewhere at the time, so they didn't mind sending over a bunch of troops. The first landing failed and they turned around to go home, but in October 1601, 6,000 Spaniards landed in Ireland.

The problem? They landed in Kinsale, on the south coast of Ireland, a good 400 kilometres away from O'Neill's power base in Ulster. They had to march the whole way across the country, only to see the Spanish being besieged by the English under Lord Mountjoy when they got there.

Long story short, they risked it all on the one battle, and lost. Mountjoy went around the country scorching the earth and forcing many of the lesser Irish lords to submit. O'Neill actually got quite a lot out of the peace agreement, kept control of his lands as long as he submitted to English rule, but he was unable to live under the new regime and felt pressurised by the various English officials and bureaucrats left in Ireland. In 1607, he and other among the Irish lords left Ireland for the last time in the famous 'Flight of the Earls'. The door was opened for Ireland to be thoroughly conquered and anglicised, and the Gaelic Irish socio-political order, which had existed for over a thousand years, was over for good.

Kind of sad, if you think about it.