Russia’s military incursion into Ukraine triggered the mass displacement of two-thirds of Ukrainian children and adolescents, creating a cascade of population health consequences and producing extraordinary challenges for monitoring and controlling preventable pediatric infectious diseases. From the onset of the war, infectious disease surveillance and healthcare systems were severely disrupted. Prior to the reestablishment of dependable infectious disease surveillance systems, and during the early months of the conflict, our international team of pediatricians, infectious disease specialists, and population health scientists assessed the health implications for child and adolescent populations. The invasion occurred just as the COVID-19 Omicron surge was peaking throughout Europe and Ukrainian children had not received COVID-19 vaccines. In addition, vaccine coverage for multiple vaccine-preventable diseases, most notably measles, was alarmingly low as Ukrainian children and adolescents were forced to migrate from their home communities, living precariously as internally displaced persons inside Ukraine or streaming into European border nations as refugees. The incursion created immediate impediments in accessing HIV treatment services, aimed at preventing serial transmission from HIV-positive persons to adolescent sexual or drug-injection partners and to prevent vertical transmission from HIV-positive pregnant women to their newborns. The war also led to new-onset, conflict-associated, preventable infectious diseases in children and adolescents. First, children and adolescents were at risk of wound infections from medical trauma sustained during bombardment and other acts of war. Second, young people were at risk of sexually transmitted infections resulting from sexual assault perpetrated by invading Russian military personnel on youth trapped in occupied territories or from sexual assault perpetrated on vulnerable youth attempting to migrate to safety. Given the cascading risks that Ukrainian children and adolescents faced in the early months of the war—and will likely continue to face—infectious disease specialists and pediatricians are using their international networks to assist refugee-receiving host nations to improve infectious disease screening and interventions.