He announced his intention when he turned 21 and assumed his rights from the regency council. Alexander sent an embassy to Norway and King Haakon IV, who rejected the Scottish claim and planned a military expedition to the islands, anticipating trouble and wanting to set up negotiations. Trouble there was, because Scottish nobles started raiding the Hebrides. In 1263, Haakon and a fleet of 120 ships—a pretty common "negotiating tactic" for Haakon—arrived in the Hebrides in August and met Alexander's forces, settling down to negotiate.
Alexander kept the talks going deliberately to reach a time on the calendar when the weather would make the sea more dangerous. An impatient Haakon attacked in October at the Battle of Largs, but a storm damaged several of his ships and the battle was indecisive. Haakon died not long after, however, and Alexander was able to assert control over the Hebrides. They were formally traded to Scotland by Haakon's successor, Magnus VI, for money in 1266 with the Treaty of Perth.
Alexander's marriage to Margaret is an interesting story. They were too young at the start to consummate the marriage, and Margaret's time in Scotland as a pre-teen was not pleasant. The marriage got better for them, however, until Margaret's death. Alexander re-married, and this second marriage was the death of him. I'll explain tomorrow.
