Who was the first president to enter into a treaty without Congressional approval?

The first president to enter into a treaty without Congressional approval was Theodore Roosevelt. He took significant steps to expand the executive branch’s role in foreign affairs, exemplified by his negotiation of the Hay-Pauncefote Treaty in 1901, which facilitated the construction of the Panama Canal. Roosevelt argued that as president, he had the authority to act decisively in matters of foreign relations, which allowed him to circumvent the typical legislative process that involves Congress in treaty-making. This set a precedent for future presidents to engage directly in international agreements, highlighting a shift towards a more assertive executive in foreign policy matters.

Although other presidents explored executive agreements or engagements without full treaties, Roosevelt's actions were particularly notable because they established the extent to which a president could exercise executive authority over foreign policy.