The Ironborn Invasion of the North was a major conflict during the War of the Five Kings in which Balon Greyjoy invaded The North after refusing an offer of an alliance from Robb Stark. Balon was initially successful in this invasion, capturing several key locations and lords. Upon Balon's death the rebellion was continued by his brother, the new king of The Iron Islands, Euron Greyjoy.
Balon ordered three main strikes on the North for his initial invasion of the North. He ordered his brother, Victarion to claim Moat Cailin, his daughter Deepwood Motte and eventually Dagmer Cleftjaw to capture Torrhen's Square. These initial assaults are successful and in addition to harrying raids on the coast, the North quickly begins to bleed.
With these victories the Ironborn began to threaten other holdings in the North. Their capture of Moat Cailin effectively stopped Robb Stark's troops from returning home lest he retake the castle.
Reclaiming The North
With his campaign in The Westerlands being a success and Tywin Lannister seemingly moved from Harrenhal to face Renly Baratheon, Robb Stark began his campaign to reclaim his homeland. He moved his Northern levies back through the neck and devised a plan to retake Moat Cailin by attacking from two fronts. Robb is successful in retaking the castle, but not after experiencing a particularly bloody battle in the Battle for Moat Cailin.
With his route back to the North secured, Robb sends hosts to expel the Ironborn from Northern lands. They are successful in reclaiming Deepwood Motte and Torrhen's Square, and upon the death of Balon Greyjoy, the Ironborn in The North begin a full retreat.
Following his victory, Robb Stark was in position to help his half-brother Jon Snow defend The Wall against the Wildling army of Mance Rayder, of which he personally led the relieving force. Following this victory, and the defeat of the Ironborn, Robb was able to direct his attention back south to broker a deal with the newly victorious, King Renly Baratheon.