Ukrainian forces’ impressive military performance has reinforced the view that an outright victory against Russia is possible. But an unqualified Ukrainian win that dislodges the Russian forces from eastern Ukraine is increasingly improbable. Grinding attrition that makes dangerous escalation a tantalizing option for both countries is more likely. Accordingly, a ceasefire and separation of forces should be a priority for the United States and its allies. The United States has the tools and experience to make it happen.