"In today's strategic environment, five key challenges - Russia, China, Iran, North Korea, and violent extremist organisations - most clearly represent the challenges facing the US military," Mattis told members of the House Armed Services Committee during a Congressional hearing last night.
"They serve as a benchmark for our global posture, the size of the force, capability development, and risk management," he said.
Over the past several decades, each of these state actors have developed capabilities and operational approaches to counter US strategic and operational centres of gravity, he said.
"Today, Russia continues to invest in a full-range of capabilities designed to limit our ability to project power into Europe and meet our alliance commitments to NATO," said Mattis.
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Similarly, Mattis said, China has embarked on a significant programme to modernise and expand strategic and conventional military capabilities.
"North Korea's nuclear weapons development, combined with efforts to develop a nuclear-capable ballistic missile capability, is specifically intended to threaten the security of the homeland and our Allies in the Pacific. Over the past year, North Korea conducted an unprecedented number of missile tests. Moreover, North Korea has demonstrated a willingness to use malicious cyber tools against governments and industry," he said.
"Finally, violent extremist organisations such as ISIS and al Qaida remain a threat to the homeland, our Allies, and our way of life," Mattis said.
A review of these five challenges demonstrates that the US military requires a balanced inventory of advanced capabilities and sufficient capacity to act decisively across the range of military operations, Mattis said.