The correct response 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 and capabilities through recent successes, which allowed for the effective use of diplomacy backed by the credible threat of force—a principle encapsulated in his famous quote, "Speak softly and carry a big stick." This approach aimed to achieve foreign policy goals through a combination of negotiation and the readiness to use military power if necessary.