The Senate unanimously confirmed the former Florida senator the previous day as the first Cabinet member to join Trump's non-consecutive second term.
Trump's primary promise "when it comes to foreign policy is that the priority of the United States Department of State will be the United States," Rubio wrote on X.
"Our foreign policy is centered on one thing and that is the advancement of our national interest..... and that will be our mission. That will be our job across the world, is to ensure that we have a foreign policy that advances the national interest of the United States," he said, stressing Trump's "America First" policy once again during his swearing-in ceremony.
"We will seek to prevent them and avoid them, but never at the expense of our national security, never at the expense of our national interests and never at the expense of our core values as a nation and as a people," he added.
"Everything we do must be justified by the answer to one of three questions: Does it make us stronger, does it make us safer, and does it make us more prosperous? If it doesn't do one of those three things, we will not do it," he elaborated.
Earlier the South Korean Embassy in Washington welcomed Rubio's nomination in November last year, expressing hopes for the "continued deepening of our security and economic ties" in a social media post.
Rubio visited Seoul in 2009, toured the Demilitarized Zone (DMZ), and emphasized the importance of the bilateral alliance between the two countries.
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