The demise of Smaug was not to be the last of Lonely Mountain's many troubles. In TA 3019 on the 17th of March, a horde of Easterling soldiers from Rhûn swarmed over the Redwater river, opening up a second front in the northern theater of the War of the Ring. The sheer force of the feared and renowned Easterlings crushed the Men of Dale and routed their forces, which had to rally back to the protection of the Lonely Mountain.
The Dwarves and Men fought a pitched battle against the invaders, eventually emerging victorious over their opponents. The impregnable gates and walls of the Lonely Mountain, furnished and extensively worked with advanced, complex and intricate Dwarven stonework, defense planning and smith-work, easily withstood the siege equipment of the Easterlings. The Lonely Mountain itself was a key strong-point and gave its defenders great tactical leverage against the attackers, being able to shoot arrows and fling stones down below in reprisal. The many years invested in improving the Lonely Mountain's defenses had paid off, and the defenders outlasted the Easterlings in the siege. The Easterlings then withdrew, suffering disproportionate casualties because of their botched campaign and their difficulty in combating the Dwarven technology, weapons, armor and defenses.