Military power, democracy, and a free market may be necessary to promote peace but none or even all are sufficient. Each of these methods has prevented wars but each has also been responsible for causing war. A defensive build-up in military deterrent forces may be misinterpreted as an aggressive build-up, a state that supports communism, socialism, or monarchy is in danger of attack by a democracy, and economic tariffs, sanctions, and trade imbalances are all susceptible to misinterpretation.
Military power in the form of deterrence can and has prevented war. Countries are reluctant to attack a military power that is capable of inflicting an unacceptable amount of damage even in defeat. In addition, if given a voice, military professionals can be valuable in preventing certain kinds of war. The military leaders will often try to avoid high-risk missions, unnecessary wars, and wars with unclear goals such as regime change. However, deterrence is not an assurance against all types of attack. A country facing large deterrent forces may launch a preemptive attack under the real or imagined threat from another. In addition, a deterrent force has no influence over countries or individuals who do not fear retaliation. Threat of death will not stop a suicide bomber.
Liberalism and democracy both include the control of political power through rule of law, political pluralism, and political competition and peaceful power transitions. While it is true there has never been a war between democratic governments, other types of governments are not secure from attack by democracies. Democracy adds political participation, universal suffrage, and the concept of universal human rights to the concepts of liberalism but adds little to the assurance of peace. Over time, democracies grow top heavy with moneyed elites and even control by corporate influence and political corruption. The U.S. attack on Iraq is an example of a democracy run amuck. Right-wing extremists distorted Wilsonian ideology to justify war as a means of spreading democracy. Corporate influences pressed for war for oil and sold it to an increasingly nationalistic public through the corporately owned media.
The economic exchange of goods and services has increased global interdependency that will do more to promote peace than either liberal governments or military power. Most modern economically advanced and advancing states recognize that war with a trade partner will result in economic loss. On the other hand, the desire to control wealth, territory, and trade routes has been the source of war throughout history.
If the world cannot learn to cooperate, we will continue to live in a state of global anarchy and with the constant threat of war. The current state of social science is not equipped to cope with the advances in science, warfare, and capitalism. As long as individuals and nations continue to identify themselves as citizens of particular geographies and flags, the world will face the danger of war. Some form of global governance that has the power to enforce the rule of law must be created and agreed upon by the peoples of the world.