Complex sentence: Despite this unexpected success, his base of support in Scotland was too narrow for a secure hold on the crown: he spent much of his'rule' appealing to Edward III for aid, or reaching out to potential Scottish allies.
Split into simpler sentences: Despite this unexpected success, his base of support in Scotland was too narrow for a secure hold on the crown. He spent much of his'rule' appealing to Edward III for aid, or reaching out to potential Scottish allies.