BLUF: This paper proposes the creation of the structures and conditions required to train, lead and deploy a cost-effective Citizen Tri-service Reserve Force at scale during periods of stretch, tension, crisis or war. The Citizen Tri-Service Reserve Force would be available for use. (1) as a third echelon front line fighting force, (2) providing rear area and lines of communication security and (3) in homeland defence, protecting critical national infrastructure.This will be achieved by establishing cadres of experienced ex Regulars and Volunteer Reservists capable of initially training and subsequently leading the Citizen Tri-Service Reserve Force. Against a changing threat the UK is likely to require more people in uniform than we have now. The Citizen Tri-Service Reserve Force concept utilises those who have already served, putting them in place to prepare thoroughly as training and leadership cadres while saving in the short to medium term the cost of recruiting, remunerating and equipping the bulk of the rank and file (90% of the workforce). It then sets the conditions to expand when required by having experienced leadership in place to build around. Strategic background The threats facing NATO in general and the UK in particular have escalated dramatically following Russia's invasion of Ukraine in February 2022. Furthermore, Eastern Europe is only one of several areas of the world where tensions have risen substantially over recent months and years; any of the others of which (eg Taiwan, Middle East) could erupt so as to distract key allies at a critical moment, potentially reducing their ability to support their NATO commitments in Europe. Additionally, depending on the attitude of individual NATO members, including a possible future US President, Article 5 of the NATO Treaty may not be quite as binding as is widely assumed. All of this needs to be set against a context in which the UK has the smallest standing army since Waterloo. Our navy and air force are similarly diminutive. Our Defence remains configured primarily for discretionary campaigning (such as Iraq and Afghanistan) and has not yet fully switched to prepare for non-discretionary conflict against a peer adversary. The current SDR provides an opportunity to address this necessary change in orientation, which is likely to require the provision of mass in uniform. Why we need the ability to generate mass As demonstrated clearly in Ukraine, in a war of any duration, units become worn out and need replacing in the line to rest and recuperate. The UK has no provision for this. Secondly, if the UK is operating in Eastern Europe in support of its NATO allies, it will have extremely long lines of communication along which its ammunition, equipment, workforce and rations will need to travel. Protecting such lines of communication needs "teeth" units, including infantry, light armour and air defence, especially against drones. Thirdly, substantial additional resources are needed to support the defence of the UK's critical national infrastructure. The outgoing Chief of the General Staff, General Sir Patrick Sanders, publicly raised the importance of the UK's ability to create military mass if required in a time of crisis in January 2024. He was referring to a citizen army (the "third echelon") which history shows is needed to win our wars; under this concept, as in both World Wars, a small core of Regulars (the "first echelon") fight to buy time; the Volunteer Reserves (the "second echelon") then reinforce and help to hold the line; and the citizen army (the "third echelon") concludes the conflict. Access to large numbers of cost-effective teams and a trained workforce from all three services would be essential in such circumstances. Warfighting assumptions always expect a war to be over within weeks rather than months or years (the size of our armed forces is indeed currently based on this assumption); history, and current events in Ukraine, shows this to be over-o