The correct answer is: Recent military successes had established the U.S.'s ability to back up its threats.
Theodore Roosevelt believed that the United States had demonstrated its military strength through recent successes, which allowed him to leverage the threat of force effectively in international relations. His approach was encapsulated in the phrase "speak softly and carry a big stick," suggesting that while he preferred diplomacy, the ability to use military force as a last resort was crucial for achieving foreign policy objectives.