Complex sentence: King Louis and his advisors were annoyed at what they considered to be an arrogant request by the Florentines, since in their recent struggle to conquer Milan, the Florentines had maintained strict neutrality despite their long record of pro-French diplomacy.
Split into simpler sentences: King Louis and his advisors were annoyed by what they considered to be an arrogant request by the Florentines. In their recent struggle to conquer Milan, the Florentines had maintained strict neutrality despite their long record of pro-French diplomacy.