The potential for further escalation in the ongoing war in Ukraine is a significant concern. Although much of this escalation will surely be directed against Ukrainian forces and civilians, horizontal escalation by Russia—that is, the act of expanding the conflict to include other actors (Morgan et al., 2008), specifically the United States and its North Atlantic Treaty Organization (NATO) allies—has become plausible, though not inevitable. It is this prospect of horizontal escalation that poses perhaps the most acute of dilemmas for U.S.
policymakers who are trying to help Ukraine while simultaneously avoiding a great power war. This Perspective describes four plausible horizontal escalation pathways. To develop these pathways in a timely manner, we did not conduct new research but instead drew from RAND Corporation expertise in Russian foreign policy, Russian
and Ukrainian military capabilities, U.S. and NATO militaries, and deterrence and escalation dynamics

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