The correct response is: Recent military successes had established the U.S.'s ability to back up its threats.
Theodore Roosevelt advocated for "speak softly and carry a big stick" as a guiding principle of his foreign policy. He believed that the ability to threaten force was a powerful tool in international relations and that the U.S. military's recent successes demonstrated the country's capability to enforce its will, thus making threats credible and effective in achieving foreign policy objectives.