A Pentagon report July 1 warned of a "low but growing" probability that the United States will get drawn into a war with the world's other major military powers, Russia and China, and predicted the consequences would be "immense."
"The probability of U.S. involvement in interstate war with a major power is assessed to be low but growing," said the 2015 National Military Strategy report.
Russia has "repeatedly demonstrated that it does not respect the sovereignty of its neighbors and it is willing to use force to achieve its goals," it said.
"Russia's military actions are undermining regional security directly and through proxy forces."
The Pentagon points to Russia's troop presence in the Ukraine conflict, though Moscow denies it has deployed its military in eastern Ukraine to bolster a separatist insurgency.
The Pentagon's last report in 2011 spoke little of Russia, but that was before Russia's annexation of Crimea last year and its backing of an insurgency which has caused the death and displacement of tens of thousands of people.
China also is singled out in the report as increasingly aggressive and threatening to U.S. security interests in a world where stability is increasingly in short supply.
China has been raising "tension" in the Asia-Pacific region, the report said, with land reclamation projects in contested areas of the South China Sea aimed at boosting China's military and civilian presence.
"Today's global security environment is the most unpredictable I have seen in 40 years of service," said General Martin Dempsey, the chairman of the Joint Chiefs of Staff, in presenting the report.
Moreover, other military powers are taking advantage of the rapid spread of sophisticated technologies to diminish the technological edge the United States once enjoyed, he said.
"Future conflicts will come more rapidly, last longer and take place on a much more technically challenging battlefield," he said. "They will have increasing implications to the U.S. homeland."
The spread of advanced technologies around the globe is eroding competitive advantages long held by the United States, such as early warning and precision strike, the report said.
The report highlights the growing nuclear and ballistic missile capabilities of Iran and North Korea, which are on a list of countries that pose "serious security concerns" to America and its allies.
Violent extremist groups like the Islamic State are coupling the use of readily available war-making technologies with their radical ideologies to destabilize entire countries and regions, the report said.
"Global disorder" is on the rise, it said.
Despite these growing threats, America's military remains the most formidable worldwide, with an annual budget of about $600 billion that dwarfs that of any other nation.
With reporting by AFP and dpa