NATO and nations face challenges regarding live training and exercises. Current and future operations are multi-national in nature, the missions and the systems are becoming more complex and need detailed preparation. Furthermore rapid adaptation to changing circumstances is needed. At the same time opportunities for live training and mission preparation are reduced due to less available resources, more peacetime restrictions and limited time span between political decision making and deployment. Mission Training through Distributed Simulation (MTDS) presents a solution to these challenges and is therefore crucial to NATO and nations' mission readiness. Despite a number of initiatives in the past to set up a NATO MTDS capability, currently NATO does not have a standing operational MTDS capability. In October 2013 the NATO task group MSG-128 was set up with the objective to establish essential elements for a permanent NATO MTDS capability for air operations and validate these elements through initial operational exercises and evaluation. The approach of MSG-128 is two-fold: 1. Define a concept of operations and reference architecture for a permanent MTDS infrastructure for air operations, including architectural requirements for integration of live components (connected flying platforms) in the MTDS architecture 2. Build the MTDS environment incrementally by executing a yearly MTDS exercise The MSG-128 concludes its work with a 4th exercise in March 2017. This exercise proves that realistic combined mission training for fighter pilots and controllers can be achieved in the initial NATO MTDS capability. This paper describes the concept of operations and initial MTDS environment that provide NATO and nations already with a current capability to conduct realistic multi-national training for air operations. The paper concludes with a path for future growth towards an effective persistent LVC environment for air operations training which is defined by the reference architecture and the requirements for integrating live flying assets.