Threats to critical infrastructure are rapidly changing. It is important to anticipate how adversaries can combine various types of attacks for more devastating effects. Executives and those in leadership need a plan to prevent or mitigate these combined attacks. This paper reviews cyber, physical and electromagnetic threats and provides a guide to incrementally protect critical infrastructure. By reviewing this information, readers should be able to understand the emerging threats, ways adversaries can combine attacks and practical steps that can be taken to reduce the likelihood of an attack. The concepts discussed in this paper apply to both existing and new infrastructure. Readers will learn about cyber defence technologies including binary hardening, contextual access control, artificial intelligence (AI), re-imaging and automated orchestration. For physical defence, readers will gain an understanding of the threats posed by snipers and weaponised drones and how to prevent or minimise their impact. To protect against electromagnetic threats, readers will learn about solar storms, directed energy weapons and electromagnetic pulses. They will learn how to use new shielding materials and devices to protect the power, cooling, telecommunication and computing systems. The information in this paper should enable readers to take immediate actions to reduce their own vulnerabilities and empower them to affect policies, design codes and laws. By influencing decision makers, readers should be able to reduce risks and lessen the likelihood and severity of an attack.