After the terrorist attacks of September 11, 2001, the international relations theory that gained the most strength was liberalism. This is because the attacks seemed to call into question the realists’ emphasis on states and their lack of attention to ideology.
Realism holds that the only important actors in international affairs are states. They believe that states have permanent interests that transcend things like ideology and religion. States act in order to increase their levels of security, not for any other reason.
The attacks of September 11 called these ideas into question. Al Qaeda was not and is not a state. Even so, its actions were extremely consequential for international affairs. Al Qaeda was motivated less by the desire for military power or territorial security and more by religion and ideology. According to realism, Al Qaeda should not have mattered and yet it did.
The rising strength of liberalism was seen to some degree in the attention paid to Al Qaeda. Another fact that shows the rise of liberalism was the invasion of Iraq. The second President Bush invaded Iraq largely because he believed that the US could make Iraq a democratic country and, thereby, increase the level of peace and security in the Middle East. This is very much in line with the liberal idea that democracy brings peace between countries.
The rising importance of Al Qaeda (a non-state actor) and the renewed popularity of the idea that we needed to spread democracy in order to bring peace to the world show that liberalism became stronger after the 9/11 attacks.