Secretary of State Tillerson defended President Trump's proposed budget cuts to his department as a reversal of unsustainably high spending in recent years.

"I think clearly, the level of spending that the State Department has been undertaking in the past – and particularly in this past year – is simply not sustainable," Tillerson told reporters while traveling in Japan.

"What the President is asking the State Department to do is, I think, reflective of a couple of expectations," Tillerson said. "One is that as time goes by, there will be fewer military conflicts that the U.S. will be directly engaged in; and second, that as we become more effective in our aid programs, that we will also be attracting resources from other countries, allies, and other sources as well to contribute in our development aid and our disaster assistance."

Trump proposes to cut State Department and other diplomatic spending by nearly a third, generating about $10 billion to offset an increase in military defense spending. That proposal drew bipartisan skepticism on Capitol Hill, even before the precise figures were released, but Tillerson argued that "historically high" spending levels can be cut if the United States cuts involvement in military conflicts around the world.

"The State Department is coming off of an historically high level of budgetary resources in the 2017 budget, and this is reflective of a number of decisions that have been taken over the past few years, in part driven by the level of conflicts that the U.S. has been engaged in around the world, as well as disaster assistance that's been needed," he said.

If lawmakers ratify Trump's proposal, the diplomatic budget will drop from $38 billion to $27.1 billion. That's lower than the $34.9 billion for combined State Department and foreign aid operations that George W. Bush requested at the end of his presidency, but still higher than the $23.9 billion Bush wanted in his first budget request.

Congressional appropriators might ignore Trump's request and try to provide more funding than he wants if the early reactions are a prologue to the spending process. "I am very concerned by reports of deep cuts that could damage efforts to combat terrorism, save lives, and create opportunities for American workers," House Foreign Affairs Chairman Ed Royce, R-Calif., said when the White House first hinted at steep State Department cuts.

Tillerson promised to make the State Department "much more effective, much more efficient" in order to work within the new budget constraints. He suggested also that the foreign policy landscape, which alarms many lawmakers, would actually make it easier to cut the budget.