United Launch Alliance pulled out of the Department of Defense competition to send satellites into space, leaving SpaceX as the only competitor.

Facing no competition, Elon Musk's SpaceX stands to receive its first contract with the Pentagon. The move also marks a historic turn as ULA, a Lockheed Martin-Boeing joint venture, has maintained a monopoly on national security space launches for a decade. ULA officials point to funds allocated under the 2015 National Defense Authorization Act as reasons why the company is skipping out on the next launch contract.

"Under the restrictions imposed by the 2015 National Defense Authorization Act (NDAA), ULA does not currently have any Atlas engines available to bid and therefore is unable to submit a timely proposal," ULA spokeswoman Jessica Rye told Defense News.

The NDAA placed limits on the use of Russian-made RD-180 engines, which power ULA's Atlas V rocket, in reaction to Russian aggression in Ukraine. The defense spending bill was set to authorize ULA to use four of the engines, however the bill wasn't signed into law in time for the company to submit its proposal.

ULA officials also point to the structure of the contract as a factor they say prevented them from remaining in the competition against SpaceX.

"It comes down to being a price-only comparison, which takes our biggest strengths off the table," ULA CEO Tory Bruno told the Washington Post.

Bruno added that he hopes lawmakers will revisit the idea of using the Russian-made engines, and that he hopes his company can compete in the future.