It occurs to me that anyone who says that democracy is inherently more peaceful than empire has never heard of the Pax Romana.
207 years of peace and order, perpetuated by what would now be considered horribly tyrannical despots. That's very nearly as long as the US has been around, period.
Even with most generous definitions of the words "order" and "peace", the peace of empires is merely competitive.
Dock from the US all the years prior to 1865, consider that its demise is a mere decades at best away, and revoke the generous definitions, and it's simply no contest.
EDIT: Did some figuring, and of all the "Paxes" (Pax Romana, Pax Britannica, etc), there were 3,207 region-years of peace generated by empire in the past 2,000+ years. Let's see democracy, already crumbling after a mere 94 years of dominance, top that.