The retired four-star Marine Corps general also spoke about how past mistakes made during battles are "a reminder that we've got to be brilliant in the basics of blocking and tackling. Right now what we want to do is be so ready and be very much aware that we fight the way we come that everybody in the world wants to deal with Secretary [of State Rex] Tillerson and the Department of State, not the Department of Defense and the United States Army."
Mattis also was critical of Iran without mentioning it by name.
"One state sponsor of terror in the Mideast cannot hide behind its nation-state status while in effect it is actually a destabilizing revolutionary regime," he said.
"The international situation is the most complex and demanding that I have seen in all my years of service — and that's over four decades," Mattis said.
However, the Defense secretary didn't specifically talk about the Iran nuclear deal, although in remarks last week to Congress said the U.S. should stay in the international agreement. Trump, who has been sharply critical of the deal, faces a looming Oct. 15 deadline on whether to certify Iran's compliance with the agreement.
At the same time, Mattis was critical of Congress for not doing enough to invest in the readiness of the military and he called on U.S. lawmakers to get rid of automatic spending cuts under budget caps and the so-called sequestration process.
"I am among the majority in this country that believes our nation can afford survival," Mattis said. "And I want the Congress back in the driver's seat of budget decisions, not in the spectator's seat of automatic cuts."
Added Mattis, "We have the time right now to prepare for war as the best way to prevent war," he said. "But should conflict break out, to get money later will not be good enough because we won't have the time at that point."
Meantime, Mattis on Monday also called for open communications with Silicon Valley and American industry to help the U.S. military get its hands on more technologically advanced equipment and capabilities. He said America's military advantage is being "eroded" as other major superpowers adopted new advances more rapidly.
"I want no longer this gulf between us to deny us the very advances that American industry is out there and executing for themselves in the private sector," said Mattis. "The advances in weaponry that are out there right now."
Mattis added, "I've lived out in Silicon Valley for the three years that I was retired. I've seen what American industry is capable of, from Silicon Valley to Michigan, from Boston to Texas. And we've got to open the communication with them much more robustly."
Elsewhere, Mattis said reforms were underway at the Pentagon to "move faster in research in engineering." He also said acquisition reforms were in the works.
Congressionally mandated legislation passed last year will result in the splitting up of the job of undersecretary of defense for acquisition, technology and logistics (AT&L) into two positions: one on the technology wing and a second on the acquisitions wing.
The Defense secretary also applauded the Army's new modernization strategy "to move us more rapidly and organize to move more rapidly."
The Army modernization plan includes acquisition reform and is expected to be formally announced this week at the AUSA convention. That said, some details of the Army plan already are known based on a memo sent last week to the general staff and from acting Army Secretary Ryan McCarthy and Army Chief of Staff Gen. Mark Milley.
The memo, which was obtained by CNBC, said the military's "recent focus on fighting wars of insurgency and terrorism allowed our adversaries to make improvements on their modernization efforts and erode our advantages enjoyed since World War II."
The Army had no comment on the memo.
Mandy Smithberger, a defense expert and director at the nonpartisan watchdog Project On Government Oversight (POGO), said to some degree that the "memo is based on a false premise that technology is going to be the deciding factor. Tactics and strategy and how weapons are used is really where the thinking needs to begin."
Smithberger believes the memo is essentially the Army's "justification for a larger budget. This is the kind of rhetoric we heard around the Future Combat Systems where the Army ended up wasting $19 billion and fielding really nothing."
The Future Combat Systems was an earlier modernization program to replace heavy armor with more networked weapon systems consisting of lighter armored vehicles, weaponized robots and other advanced tech on the battlefield. It was formally ended in 2009 but information was still learned from the program that some experts see as beneficial today.
The Army memo stated the need to streamline work "to overcome the bureaucratic inertia and stove-piping found in the Army's current construct. It will directly incorporate requirements from the warfighter into the acquisitions process and allow us to prototype concepts. It will enable disruption — the messy, chaotic work that is the hallmark of truly innovative organizations."
According to the document, the Army "must turn ideas into action through continued experimenting and prototyping, improving acquisition business process, pursuing appropriate/off-the-shelf options, and improving training."
Indeed, some members of Congress also have pushed for the Pentagon to use more commercial off-the-shelf products as a way to cut costs in the military.
Some consumer electronics already have found their way into the U.S. military. For example, Microsoft's Xbox 360 game controllers have been used to operate Navy submarine periscopes. Also, other branches of the armed forces have used similar off-the-shelf devices to control robot movements.
POGO agrees with the use of off-the-shelf options "as long as it's something that is truly commercial," said Smithberger. However, she said there are "a number of instances where the military has purchased things claiming that it's commercial off the shelf but there's so many modifications made that it really isn't commercial — and costs get totally out of control."