The Obama administration last month sent Congress a 2016 budget request that seeks $561 billion in baseline national defense funds, $38 billion over existing federal spending caps. Unless Congress acts, the Pentagon would get just under $500 billion in fiscal 2016 after sequestration's ax does its work.
That amount of funding, Carter and Joint Chiefs Chairman Gen. Martin Dempsey told the Senate Armed Services Committee, means the military would be unable to carry out the current national defense strategy.
"This committee and this Congress will determine whether our troops ... can continue to defend our nation's interests around the world with the readiness, capability and excellence our nation has grown accustomed to, and sometimes taken for granted," Carter said.
"Halting and reversing the decline in defense spending imposed by the  Budget Control Act, the president's budget would give us the resources we need to execute our nation's defense strategy," Carter said, meaning an un-sequestered funding level equal to Obama's $534 billion request for the military.
"It would ensure we field a modern, ready force in a balanced way, while also embracing change and reform, because asking for more taxpayer dollars requires we hold up our end of the bargain — by ensuring that every dollar is well spent," the secretary said.
"The president is proposing to increase the defense budget in fiscal year 2016, but in line with the projection he submitted to Congress last year in the fiscal year 2015 budget's Future Years Defense Program [FYDP]," Carter said.
"The Defense Department needs your support for this budget," he told the panel, "which is driven by strategy, not the other way around."
SASC Ranking Member Sen. Jack Reed, D-R.I., warned of a coming "budgetary train wreck in the United States."
"With each passing year since the BCA was enacted in 2011, and with the United States slashing its defense spending as a result, the world has become more dangerous, and threats to our nation have grown," McCain said. "I don't think that is purely a coincidence."
McCain, Reed and House Armed Services Committee Chairman Rep. Mac Thornberry, R-Texas, are pushing the chamber's Budget committees to provide more Pentagon spending than the caps would allow.
While McCain agrees with Obama's topline number, he made clear he intends to push back and reverse some of its specific proposals.
"The president's budget request responds to many critical priorities, particularly addressing cyber and space vulnerabilities, military readiness shortfalls, and essential long-term modernization initiatives," McCain said.
"At the same time, the president's request reflects budget-driven policy decisions that would reduce some critical military capabilities, either through the early retirement or cancellation of existing systems, deferred development or procurement of new systems, or withheld funding for proven requirements," he said. "This committee will closely scrutinize these decisions and seek to meet urgent and legitimate military needs where possible."
The new secretary said "getting it together" to address sequestration "is a matter of deterrence."
Meantime, Dempsey told the panel he believes he believes the US should "consider" providing lethal arms to Ukraine in its standoff with Russian-backed separatists.
On the Islamic State, Carter called the group "the social media terrorists." He said the US and its allies "have to take the steam out of this thing."
"They're not invincible," Carter said.
And on Russia, Carter noted that President Vladimir Putin "talks openly" about "having countries around him that are in his orbit, adding "Ukraine is an example of that." He warned that "Putin will keep pushing and pushing."