The 18th-century German military strategist Carl von Clausewitz defined war as the continuation of politics by different means and, like the ancient Chinese strategist Sun Tzu, believed that securing peace meant preparing for violent conflict. As the world becomes increasingly tumultuous — apparent in the revival of military struggle in Ukraine, continued chaos in the Middle East and rising tensions in East Asia — such thinking could not be more relevant.

Wars are traditionally fought over territory. But the definition of territory has evolved to incorporate five domains: land, air, sea, space and, most recently, cyberspace. These dimensions of "CLASS war" define the threats facing the world today. The specific triggers, objectives and battle lines of such conflicts are likely to be determined, to varying degrees, by five factors: creed, clan, culture, climate and currency. Indeed, these factors are already fueling conflicts around the world.

Religion, or creed, is among history's most common motives for war, and the 21st century is no exception. Consider the proliferation of jihadist groups, such as the Islamic State, which continues to seize territory in Iraq and Syria, and Boko Haram, which has been engaged in a brutal campaign of abductions, bombings and murder in Nigeria. There have also been violent clashes between Buddhists and Muslims in Myanmar and southern Thailand, and between Islamists and Catholics in the Philippines.