Realpolitik and rationalism are not the same thing. Realpolitik is the idea that politics (esp. international politics) is ultimately dominated by power politics where each actor is trying to maximize their power and self-interest, and is generally an idea of the realist school of politics

While that might arguably be compatible with liberalism within a society where there is a relatively benevolent monopoly of power imposing a certain number of rights and rules molding the self-interest of all in such a way as to be compatible with civilization, within the international system there is no monopoly of violence, and therefore norms and rights are contingent on countries not acting purely in their self-interest and therefore to maximize their power.

Since modern liberalism is inherently internationalist (arguably since Woodrow Wilson, some called his approach to diplomacy "idealpolitik"), modern liberalism does have to reject realism as a school of thought.

The debate between liberalism and realism as it pertains to international politics is the main topic of contention in international relations theory.

More generally, you can also argue that liberalism assumes the good faith of powerful people not to destroy the liberal system to maximize their own power, but this is far easier to argue against in the domestic setting than in the international setting.