Final answer:
The Senate ratifies a treaty as part of the checks and balances system to check the Executive Branch. This process requires a two-thirds vote in the Senate and is a part of the legislative branch's advising and consenting role established by the U.S. Constitution in Article II.
Step-by-step explanation:
The branch being checked when the Senate ratifies a treaty is the Executive Branch. According to the U.S. Constitution, particularly Article II, the process of treaty ratification is a critical aspect of the system of checks and balances that governs the relationship between the legislative and executive branches of government. While the president has the power to negotiate treaties with foreign governments, it requires a two-thirds vote of the Senate for these treaties to be ratified. This requirement ensures that the Senate has an advising and consenting role in the process, providing a means to check the powers of the Executive Branch. The historical context dates back to 1789, during the presidency of George Washington, where the Senate's role was solidified after considering the treaty with the Southern Indian tribes. Over time, the treaty-making process has evolved, with executive agreements sometimes being used instead of formal treaties, but the Senate's role in ratifying significant international treaties remains a fundamental component of U.S. foreign policy.