The first year of the war in Ukraine has presented critical lessons for the UK's Defence Medical Services (DMS) regarding its preparedness to support the nation for warfighting at scale. There are tactical, clinical, and strategic challenges that must be addressed. The war has exposed the limitations of international humanitarian law and the laws of armed conflict in protecting forward field hospitals from deliberate targeting. The DMS may need to employ measures such as disguise, deception, and dispersal to provide care in a contested environment. The historical trend of disinvestment in military medical capability between major conflicts, known as the "Walker Dip," represents a clinical risk that must be mitigated. Even if this is achieved, clinical outcomes during large-scale warfighting are likely to be worse that those the nation has come to expect during more low-intensity conflicts. Effective civilian-military collaboration will be paramount to manage casualties at scale. Both novel and reversionary modes of transportation may be required, such as the mass movement of casualties by train. The need for a sufficient and capable medical workforce, amid global shortages and post-COVID burnout, calls for further investment. The DMS requires innovation and adaptability to harness the ability to adopt external ideas, translate successful innovations and address complex challenges. By addressing tactical vulnerabilities, enhancing clinical preparedness, fostering civilian-military collaboration, and embracing innovation, the DMS will be better equipped to support the UK and allied armed forces in future warfighting at scale.