Throughout his reign, Alfonso contended with the nobles, particularly the families of Nuño González de Lara, Diego López de Haro and Esteban Fernández de Castro, all of whom were formidable soldiers and instrumental in maintaining Castile's military strength in frontier territories. According to some scholars Alfonso lacked the singleness of purpose required by a ruler who would devote himself to organization and also the combination of firmness with temper needed for dealing with his nobles although this is not a view taken by all. Others have argued that his efforts were too singularly focused on the diplomatic and financial arrangements surrounding his bid to become Holy Roman Emperor.