Mattis was joined by Air Force Gen. Paul Selva, vice chairman of the Joint Chiefs of Staff, at the hearing of the House Armed Services Committee, which focused on the recently released Nuclear Posture Review and the National Defense Strategy.
The 2018 Nuclear Posture Review outlined "tailored strategies" for dealing with Russia, China, Iran and North Korea and called for the development of a new "low yield" submarine-launched ballistic missile and a new sub-launched cruise missile.
"Our competitive edge has eroded in every domain of warfare: air, land, sea, cyber and space," Mattis said, adding that insufficient funding has created an under-resourced military.
Mattis said the United States would invest in new technologies like nuclear deterrent forces, missile defense and artificial intelligence to help prepare the armed forces.
"If you threaten us, it will be your longest and worst day," Mattis told the panel. "To implement this strategy, we will invest in key capabilities recognizing we cannot expect success fighting tomorrow's conflicts with yesterday's weapons and equipment."
The defense chief said the second line of effort would be to strengthen traditional partnerships with U.S. allies.
Mattis also cited great power competition as the highest priority and primary focus of national security -- citing China and Russia as two countries that are a "threat" to the U.S.'s military capabilities.
Russia, China and Iran have criticized the U.S. nuclear review as something that would only encourage nuclear weapons proliferation. They called it "confrontational" and "anti-Russian," while Iranian Foreign Minister Javad Zarif said the review risked bringing "bringing humankind closer to annihilation."
Mattis said the United States is committed to reducing its nuclear weapons stockpile, but said deterrence in arms control can only be achieved with a "credible capability" -- adding that Russia and China have been modernizing their weapons and nuclear systems.
"We are going to stay inside the Intermediate Nuclear Forces Treaty compliant requirements but we are going to do research & development of an alternative weapon that should put Russia in a position to see the value to returning to being INF compliant."