The centerpiece of the budget signed by President Donald Trump is a $61-billion increase in defense spending, boosting funding for the government's biggest department to $700 billion.

That is more than half of the government's projected total spending for the current fiscal year.

Since taking office, Trump has frequently touted his support for the US military and placed high-ranking generals in top White House and cabinet posts, though not all have survived.

National Security Advisor HR McMaster, a lieutenant general still serving with the army, became the latest top official to go when he announced on Thursday he is leaving the post.

Along with Secretary of State Rex Tillerson, who Trump fired on March 13, McMaster and Mattis jointly resisted Trump's instinctual rejection of multilateralism.

Of that trio, only Mattis remains. The defense secretary stood with Trump and a few other cabinet members on Friday to publicly discuss the budget, which Trump called "a record" for defense.

Accounting for inflation, the 2018 Pentagon budget is still less than that of 2007, which was the equivalent of $785 billion.

Trump said he had considered vetoing the entire $1.3-billion budget but held back "because of the incredible gains we've been able to make for the military."

Despite voicing displeasure with many items included in the budget bill, Trump said "as a matter of national security" he approved it.

"It increases total defense spending by more than $60 billion from last year and funds the addition of critically needed ships, planes, helicopters, tanks and submarines," the Republican commander-in-chief said.

While Tillerson and McMaster frequently became public targets of Trump's anger, the Pentagon chief remains in good stead -- even if he frequently disagrees with the president.

Mattis has proven able to coax his boss away from inflammatory rhetoric and towards more traditional US positions.

His influence was reportedly crucial in the tense lead-up to the budget's signing.

Fox News reported that Mattis called the White House to warn about losing the extra defense spending because of a veto, and then personally visited 1600 Pennsylvania Avenue mid-Friday morning.

To avoid friction, Mattis has studiously avoided much media coverage -- Trump has shown clear dislike for members of his team who pull attention away from him.

On Friday, when Trump invited his Pentagon chief and some other cabinet members to speak briefly, Mattis took his turn, thanking Trump and promising to use the money wisely.

"As the president noted, today we received the largest military budget in history, reversing many years of decline and unpredictable funding. And together we are going to make our military stronger than ever," Mattis said.

"We in the military are humbled and grateful to the American people for their sacrifices on behalf of this funding."