From Seth G. Jones
"IW refers to activities short of conventional and nuclear warfare that are designed to expand a country’s influence and legitimacy as well as weaken its adversaries."
While conventional warfare—set-piece battles between large military forces—largely defined twentieth-century conflict between major powers, irregular warfare will likely define international politics over the next year and beyond. Countries like China, Russia, and Iran compete with the United States using irregular methods because conventional and nuclear warfare are far too costly. The tools of irregular warfare are not strategic bombers, main battle tanks, or infantry soldiers, but hackers, intelligence operatives, special operations forces, and private military companies that often operate in the shadows.
Full Report: https://www.csis.org/analysis/future-competition-us-adversaries-and-growth-irregular-warfare