Battle of Dale
The Dwarves of the Lonely Mountain and the Men of Dale refused to acknowledge the Overlordship and alliance of Sauron. While his southern armies menaced Gondor, he sent an army north to extend his dominion to prevent the armies of his enemies joining together under one banner, which could have proved disastrous for Mordor.
On March 17th of the year 3019 in the Third Age, Sauron sent a large contingent of Easterlings to attack Dale. The combined forces of the Men of Dale under King Brand and the Dwarves of the Lonely Mountain under King Dáin Ironfoot marched out to meet the Easterlings in battle. Sauron's forces were probably more numerous, though the army of Dale would have possessed an advantage due to their superior Dwarven-made weaponry. After three days of heavy close-quarters fighting, they were forced to retreat to the Lonely Mountain.
A few sturdy warriors led by Brand and Dáin fought bravely before the Gate of Erebor, which was not taken. In the end, Dáin was killed as he stood defending the body of his ally Brand. Meanwhile, the defenders of the Mountain were able to withstand the siege.
However, to the misfortune of the Easterlings, the forces of Gondor and Rohan defeated the main power of Sauron in the southern theatre on March 25th, causing the northern army to lose heart. Seeing the morale of their foes being sapped by news of victory in the south, the Army of Dale under the new Kings — Bard II and Thorin III Stonehelm — managed to lift the siege on March 27th and drove the Easterlings out of Dale.
The battle was incredibly important in the course of the War of the Ring: if Sauron's Easterling armies hadn't had the Dwarves and Men of Dale to fight, they would have been able to join up with Sauron's forces from Dol Guldur in their attacks on the Woodland Realm of Mirkwood and Lothlorien, tipping the scales in favor of Mordor. This would have enabled the Mordor-armies to flank the forces of Gondor and Rohan from the North and rear. Gandalf himself commented that had the Battle of Dale been lost this way, the forces of the West would have been crushed regarless of the victory at the Battle of the Pelennor Fields.
The Battle of Dale would have further reduced the already low numbers of Dwarves and paved way for the dominion of Men in the Fourth Age.