How did Constantine I become emperor after Constantius I died in Britain?

Following the death of his father, Constantius I, in 306 CE at Eboracum (modern York), Constantine I became emperor through the immediate acclamation of the troops stationed in Britain and Gaul.12

This action bypassed the established rules of the tetrarchy, a four-man ruling system created by Diocletian that dictated the succession process. Although the senior emperor Galerius had intended for Flavius Valerius Severus to govern the West as augustus, he was forced to grudgingly concede the subordinate title of caesar to Constantine following the army's declaration. This event initiated a period of political instability and civil war, as various claimants vied for imperial power.1342