The impacts of AI are growing within a wide range of spectrum including the national security and defense. AI has a direct potential impact on the most critical elements such as surveillance, intelligence, threat detection, and secure communications. The security threats have increased majorly from the linear battlespace to a wide range of actions such as cyber warfare, hybrid operations, and information manipulation campaigns. The traditional defense systems are not well equipped with these threats. So, AI can help in the prediction of emerging threats and also help the decision-makers with fast and actionable strategies. When considering the trustworthiness of AI in national security and defence, trust is paramount. Defence agencies must know that their systems are not only effective but also secure, reliable, explainable, and scalable. Consequently, implementing trustworthy AI means being resilient to attacks, transparent about decisions, ethical to use, and secure at scale. In this chapter, we will examine four examples of AI as a shield and a force multiplier to obtain operational advantages in secure communications, resilient telecommunications networks, cyber defence, and mission assurance operations.