Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE on Sunday offered his support for the Trump administration's proposed "Space Force," and defended his stance after having urged lawmakers to oppose it a year ago.

Speaking to reporters on board a flight to Brasilia, Brazil, Mattis insisted he initially opposed the creation of a separate military branch for space because the administration had not yet defined the challenges space posed for the Pentagon.

"I was not going against setting up a Space Force; what I was against was rushing to do that before we define those problems," Mattis said. "We've had a year, over a year in defining. And the orbitization of this solution in terms of institutionalizing forward momentum is very important."

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His comments come days after Vice President Pence visited the Pentagon to announce the first steps in the creation of the Space Force — despite the fact that Mattis himself had opposed the move when President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE announced his plans to move forward with the proposal.

Mattis in July 2017 wrote a letter to Rep. Michael Turner (R-Ohio) calling on lawmakers not to go forward with Trump's proposal.

“I strongly urge Congress to reconsider the proposal of a separate service Space Corps,” Mattis wrote. “I believe it is premature to add additional organizational and administrative tail to the Department at a time I am trying to reduce overhead.”

Turner had been pushing for a revision to the National Defense Authorization Act (NDAA) that would have removed a provision creating Trump's Space Force and replaced it with a requirement for the Defense Department to instead study the proposal.

But last week, prior to Pence's visit to the Pentagon, Mattis insisted he supported the creation of a Space Force and said the plan was “in complete alignment with the president's concern about protecting our assets in space.”

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On Friday, Deputy Defense Secretary Pat Shanahan said Mattis's initial opposition had to do with budgetary concerns.

“When [Mattis] made those comments a year ago, right, the timing is very important, because we were facing another [continuing resolution], we had budget caps ... and we were going through significant belt-tightening exercises,” Shanahan said.

But other defense officials, including Air Force Secretary Heather Wilson, had also expressed opposition to the move last year, arguing it would impose unnecessary bureaucracy on the Pentagon.

Mattis on Sunday acknowledged that Congress will bear much of the responsibility for initiating the creation of a Space Force. But he said there are things the Pentagon will be able to do on its own.

"There are things we can move out on immediately," he told reporters. "Actually, we already have been, and all this does is give that an emphasis, and give us added support from above that we then take full advantage of, in terms of funding organization, and re-organizing so that we have more — I would call it more cohesion."

Mattis is traveling across South America to discuss various defense-related issues. He is expected to visit Argentina later in the week, the first visit by an American defense chief to the country in more than a decade.