An outbreak of SARS CoV-2 infection occurred in an infantry battalion from Joint Base Lewis-McChord following participation in a field training exercise in the vicinity of Yakima, WA in February of 2021. Extreme weather during the exercise disrupted planned COVID-19 mitigation measures and caused 110 soldiers to be sheltered in a small aircraft hangar for several nights. The probable index case reported to sick call with symptoms compatible with COVID-19, but the soldier was not diagnosed with COVID-19, was returned to duty, and was allowed to remain in the enclosed hangar for 3 additional days. In total, 143 individuals with epidemiologic ties to the field training exercise tested positive for SARS-CoV-2 during the outbreak. Nine samples sent for sequencing were determined to be the SARS-CoV-2 Beta variant. This report illustrates important lessons learned whose implementation in the future will enable better protection of service members from COVID-19 and similar health risks associated with training.