The U.S. now has about 8,400 troops in Afghanistan and North Atlantic Treaty Organisation and coalition allies have an additional 5,000.
The warnings came as Trump was reportedly considering whether or not to send additional hundreds of USA troops to Afghanistan.
Mattis and Chairman of the Joint Chiefs of Staff General Joseph Dunford testified before the Senate Armed Services Committee on Tuesday, discussing how the Pentagon's budget will be used to address a myriad of issues, from troop use in Afghanistan and Syria to media reports of cyber threats from Russian Federation.
Mattis appeared alongside Marine Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, and Pentagon comptroller David Norquist.
The secretary said the fiscal 2018 budget reflects five priorities, the first being to continue to improve warfighter readiness.
"Four years later, I returned to the department and I have been shocked by what I've seen with our readiness to fight", the defense secretary said. "No enemy in the field has done more to harm the readiness of our military than sequestration".
"It makes it hard to support you when we don't have a strategy", McCain said at a committee hearing discussing the Pentagon's 2018 budget request.
Reuters reported in late April that the administration of US President Donald Trump was carrying out a review of Afghanistan, and conversations were revolving around sending between 3,000 and 5,000 US and coalition troops there.More news: Sony's new PlayLink for PS4 will let users play games in groups
In this picture relased by the US Department of Defence, US Army Soldiers from 1st Battalion, 41st Infantry Regiment, 3rd Brigade, 1st Armored Division scan the landing zone for possible hostile forces 26 August 2005. Mattis added that the plan would require additional money that's not in the current budget request. That includes $574 billion for the Pentagon "base budget" and almost $65 billion for overseas operations.
Mattis, under questioning from Sen.
Rep. Bradley Byrne, R-Ala., the Seapower Subcommittee's vice-chair, touted estimates it would take another $5 billion per year to reach a 355 Navy, and asked Dunford when the administration would "get to that point".
They said this will be separate from the request to increase the number of advisers for training and assistance and the USA would expect to increase the frequency of airstrikes launched. Who's left in the lurch?
A suspected North Korean drone had photographed a USA missile defense shield in South Korea before it crashed near the border where it was found last week, the Defense Ministry said Tuesday.
Being able to get military capabilities where they need to be and sustaining them once there plays an important part in reassuring allies around the world, the chairman said.
This mandates new investment, innovative approaches and new program starts - something denied under continuing resolutions, Mattis said.
The GOP-led Congress rebuffed the Obama administration's requests to reduce the number of military bases.