Donald Trump attracted attention with his sensational statements and anti-establishment attitude during his tenure as president of the United States. This anti-establishment attitude was not limited to its own country, but also reached a point where it would wage war on the established practices and rules of international relations and law. Americanism, which comes to life in the slogan of “America First”, has gained a strong ground against multilateralism with the policies of the Trump administration. In this study, the anti-establishment actions of the Trump administration against the international legal system are discussed. In this respect, the study is divided into two main parts. In the first one, it is shown with examples that the Trump administration easily withdrew from international agreements and organizations that apparently did not comply with the interests of the U.S., it mandated arrangements in its favor with the threat of withdrawal, it used its financial contributions to international organizations as a weapon, and it refused international participation and “soft law” tools in any situation which was against its interests. In the second part, it is also shown with examples that the Trump administration either directly violated some of the basic rules of international law or condoned the violation of them and carried out attacks on international institutions at the level of rhetoric and action. Three main claims are made in the study. First, by violating the fundamental principles of international law, the Trump administration has seriously damaged the US’s notions of multilateralism and internationalism. Second, Trump’s approach was based on his own national interests, not the universal rights that form the basis of modern international law. Third, the Trump administration not only violated international law, but weakened it by challenging international legal norms and institutions when U.S. interests demanded so.