By Brooke Singman, Blake Burman | Fox News

President Trump is signing an order Thursday that imposes tariffs on steel and aluminum imports from all foreign countries, while carving out an exception for Canada and Mexico for now while North American Free Trade Agreement (NAFTA) negotiations are under way, White House officials told Fox News.

The president, joined by steel and aluminum workers, will sign a companion proclamation — one on steel and one on aluminum – that will institute a tariff of 25 percent on steel and 10 percent on aluminum imports. The tariffs will become effective in 15 days, with exclusions for Mexico and Canada taking effect “immediately,” officials said.

“A nation that does not protect prosperity at home cannot protect its interests abroad,” the president said. “When it comes time where our country can’t make aluminum and steel, you almost don’t have much of a country.”

A senior White House official said Thursday that the move to impose tariffs would protect “national security” from the effects of “unfair trade practices.”

The U.S. is the world’s largest importer of steel, importing nearly four times as much as it exports, according to the White House. The U.S. imported five times as much primary aluminum as it produced in 2016.

The senior official told reporters Thursday that the move came after an investigation, led by Commerce Secretary Wilbur Ross, and interagency meetings from April 2017 through January 2018, which found that the current level of aluminum and steel imports to the U.S. had the “potential to threaten our national security.”

“The investigation provided the president with the powers to take actions, should the department find that imports of commodities like aluminum and steel provide a significant threat to our national security and economic security,” the official said, noting that national security was “broadly defined,” and not limited to national defense.

Ross recommended a 24 percent tariff on steel, but the administration official said that the official 25 percent tariff was based on additional data and support.

Aluminum is used in a range of ground weapons and aircraft, and steel is required for aircraft carriers, amphibious force ships, submarines, tanks and light armored vehicles. The White House said the investigation rendered the U.S. “unable” to produce enough steel and aluminum to meet national defense and critical industry needs in the event of a national emergency.

The move though, should not be a “surprise,” according to the official, who called it another “promise made” and “promise kept” by Trump.

“No one should be surprised with any kind of action we’re taking,” the official said. “This is one of the most well-discussed, well-signaled actions in the history of trade actions.”

The Trump administration, in January, approved safeguard tariffs on imported large residential washing machines and solar cells.

The official said Thursday that there is language in the proclamation outlining “security relationships” with countries around the globe, and noted that “they will be welcome to discuss” with the U.S. “alternate ways” to tariff the imports.

Officials said that Mexico and Canada would be exempt from the tariffs during NAFTA negotiations. The president, himself, has hinted at other exemptions.

“If that [NAFTA] negotiation is unsuccessful then tariffs will be applied across the board,” Peter Navarro, the director of the White House National Trade Council, told Fox News. “Now, importantly, in addition to that, there is language that will allow other countries to effectively propose ways that they could get a similar dispensation in exchange for more fair and reciprocal trade with the United States.”

But the 25 percent and 10 percent levels should not be seen as firm, officials said. Navarro added that if Canada, Mexico, or any other country is eventually granted an exemption, then tariffs would be raised for all other countries affected.

The president, himself, has said the U.S. would be “very fair.”

“[We’re] sticking with 10 and 25 percent initially,” Trump said during a Cabinet meeting on Thursday. “I’ll have a right to go up or down, depending on the country.”

Trump added, though, “We’re going to be very flexible.”

Navarro told Fox News that the president was not using the tariffs as a “bargaining chip,” but noted that “any country that is allowed to be removed from the tariffs would result in higher tariffs for everybody else.”

“So we still maintain a level of protection in defense of the aluminum and steel industries,” Navarro said.

The White House said that “key sectors” of steel and aluminum industries have faced “long-term downward trends in employment.” The White House said employment in iron and steel mills has dropped by more than 54,000 since 2000. The aluminum industry has cut more than 40,000 jobs in the same time period.

“The decline of American steel and aluminum production has resulted in extensive job losses for hardworking Americans in industries critical to our national security and economic well-being,” the White House said Thursday, noting that despite “considerable growth in demand,” six primary aluminum smelters have permanently shut down since 2012.

Fox News’ Adam Shaw, John Roberts, Kristin Brown and Serafin Gomez contributed to this report.