  Lewis Fry Richardson argued that the frequency and severity of deadly
conflicts of all kinds, from homicides to interstate wars and everything in
between, followed universal statistical patterns: their frequency followed a
simple Poisson arrival process and their severity followed a simple power-law
distribution. Although his methods and data in the mid-20th century were
neither rigorous nor comprehensive, his insights about violent conflicts have
endured. In this chapter, using modern statistical methods and data, we show
that Richardson's original claims appear largely correct, with a few caveats.
These facts place important constraints on our understanding of the underlying
mechanisms that produce individual wars and periods of peace, and shed light on
the persistent debate about trends in conflict.
