The No. 2 official at the Pentagon said Friday that 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 had earlier urged Congress to not create a branch of the military focused on space because of budgetary concerns.

Mattis wrote in a letter to lawmakers in July 2017 that creating a new branch of the military dedicated to space would be “premature and add additional organization and administrative tail to the department at a time I am trying to reduce overhead.”

However, the secretary offered support for a Space Force on Thursday, after 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 repeatedly spoke in favor of establishing a sixth branch of the military and ordered its creation earlier this year.

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Deputy Defense Secretary Pat Shanahan cast Mattis's reticence over the Space Force as being related to the "sequester," which imposed tight budgetary ceilings on the Pentagon.

“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 told reporters on Friday.

Mattis had written in his letter last year that creating a “Space Corps,” as the concept was titled at the time, would force the Defense Department to work on creating a new military branch before they had time to study it.

“I share congressional concerns about the organization and management of the department’s space capabilities,” Mattis wrote at the time.

“The creation of an independent Space Corps, with the corresponding institutional growth and budget implications, does not address the specific concerns nor our nation’s fiscal problems in a responsive manner.”

Shanahan did not address the other concerns raised by Mattis's letter in saying why the defense official initially opposed creating a space division of the military.

The Defense secretary was supportive of Trump’s Space Force earlier this week, saying the Pentagon is “in complete alignment with the president's concern about protecting our assets in space.”

Mattis declined to say whether a combatant command he said the Pentagon was working on would be the same as creating a separate branch of the military.

He added that he, Trump and Vice President Pence “are in complete agreement.”

The law imposing the sequester on the Pentagon is still in place and would impose new limits on defense spending in fiscal 2020, when the administration hopes to begin the Space Force.