According to liberal theories of international relations, what are the causes of war between states. According to their perspectives, will there be war between the US and China in the twenty-first century?

Liberalism in international relations theory is the more sanguine counterpart to realism (or "power politics"). It rejects the notion that the relative power of a state is the determining factor for whether a state goes to war. Liberal theory holds that states very often behave (and have reason to behave) in ways that are contrary to how it might in a unmitigated power struggle. Circumstances that affect whether a country chooses to go to war include the following: (1) whether that country is a democracy, (2) the status of free trade among the different states, and (3) the existence of international institutions whose aim is to normalize trade and promote a shared value system (such as the UN). If one or more of these circumstances are not present, nations are more likely to engage in war.
China can rise peaceably according to a liberal theory of international relations. In fact, many people cite the rise of China as evidence to support a liberal view of international relations, as the country's increased power owes to economic expansion, itself the result of liberal trade policies. China has sought more interdependence in its economy and has begun to view the US as a trading partner and primary supplier of goods and technology, rather than an inimical competitor. Finally, China became a member of the World Trade Organization (WTO) in 2001, so it is bound to a major international institution. The economic nature of China's rise to power, coupled with its slow but promising transition to a democratic structure of government (and attending democratic ideals), suggest that there will not be a war between the US and China according to liberal theories of international relations.