Treaty of Dublin (The Arrow That Missed)

The 1656 Treaty of Dublin was the document that originally founded the UK and formally ended the HRE, which was devolving into chaos after nearly 50 years of non-stop civil war. In exchange for Austrian and Anglish permission to leave the HRE peacefully, as well as Anglish help against Scandanavia, Brandenburg ceded the Netherlands (the province of Loither) to the Anglish, while Austria recieved Brandenburgish help in the war on Bavaria. One provision that both Brandenburg and Angland insisted on was the incorporation of Eire into the UK. Eire at that point was also in civil war, with the King recently killed, and the Regent nearly powerless. Despite being firmly against the deal, as they felt this would throw the balance of power off, Austria was at the time locked in a massive war with Bavaria, as well as a looming one with Spain, and urgently needed Brandenburgish help with these. Brandenburg, meanwhile, was firm on this point, as they needed to destabilize their Scandanavian enemies, who were close allies with Eire. At the same time, Angland would then enter the war, allowing them to win.

Angland
Following the disastrous war with Brandenburg in 1583-6, Angland's empire collapsed, being reduced just to the Island of Britain. Since then, Normandy and France had been reclaimed, down to Poitiers, as had Iceland, but Angland was now in the middle of a political crisis, with the army barely able to hold what they had, as well as public outcry against incompetent rulers. Many urged that the province of the Netherlands (including all of the Low Countries) should be attacked and annexed, as this would also bring in large amounts of gold, but King William III remained wary of engaging in another war against Brandenburg. Still, without some new gains, it was feared that the great Anglish Empire would never rise again.

Austria
Austria was also in a desperate crisis. Having spent the last 100 years fighting enemies within and without the HRE, it's finances were in ruins and it's army exhausted. Meanwhile, it was fighting a losing war against Bavaria and it's Schismist allies, while the north had started to secede. Saxony had seceded almost 15 years ago, but Austria had been unable to stop it due to Brandenburgish interference; effectively, had Austria attempted to attack Saxony all of North Germany would secede and rip the HRE apart. With the war in Bavaria, it was acknowledged that it would be impossible to fight on both fronts at once. Since then, though, most of the north had seceded, and now Brandenburg was attempting to, only stopped by the fear of Angland intervening with Austria, it's old ally. Nevertheless, with the war in Bavaria having now reached new lows, and the Spanish split from Austria (which was alomst certain to lead to war), Austria needed a way out from this struggle.

Brandenburg
Brandenburg, despite seeming to have the upper hand, was in fact in worse shape than either of the others. The HRE civil war, the Polish wars, and now the Scandanavian War had chipped away at Brandenburg's world standing and finances. The Russo-Norse union fighting Brandenburg was almost certain to defeat them, possibly even to the point of annexation, and with parts of it's provinces in open rebellion- including the Netherlands- it was on the point of collapse. it needed aid against Scandanavia and Russia, a way to leave the HRE without sparking yet another war, and a way to quell the rebellions in it's provinces.

Eire
Eire was united into a single union in 1486, but after a brief period of peace, the country plunged into civil war in 1542 and hadn't emerged out of it yet. When the last King died, in 1611, a Regency was put in place. over the last 50 years, though, all it succeeded in doing was losing all the land in Eire except that between Dublin and Belfast. Now, one of the Earls in Central Eire had managed to get a stable army up, and might even succeed in destroying the rest of Eire. The man, known as a warlord and a brutal tyrant, had complete control over his army and was marching northwards. In the region, only the Anglish could defeat him, and if that meant annexation, well, so be it.