American ships and fighter jets manoeuvring across the South China Sea and the Sea of Japan represent the "new normal" in US-Pacific relations despite rising tensions with China and Moscow.
US moves in recent months have led to angry protests from China and Russia, which contend the Obama administration is fuelling unrest in the Asia Pacific and conducting illegal and unsafe transit in the region.
US military leaders defend the operations and say they will continue to exercise freedom of navigation, and may do so more frequently as time goes on.
The escalating rhetoric reflects efforts by China and Russia to show military superiority in an increasingly crowded and competitive part of the world.
And it sets up a tense game of political brinkmanship as leaders from the two countries and the US thrust and parry across the military and diplomatic fields of play.
The military manoeuvres have shadowed President Barack Obama's "pivot to Asia," a decision early in his tenure to try to focus the relationship with Pacific partners on economics and trade.
"We're at a moment when China, Iran and Russia are all testing us, engaging in reckless behaviour and forcing policy makers with the question of how far we push and when," said Derek Chollet, a former assistant defense secretary for international affairs and now a senior adviser at the German Marshall Fund.
"We're for freedom of navigation and following the rules, and to an extent we are pushing back against changing the rules."
Adm John Richardson, chief of naval operations, said that for the first time in 25 years, the US is facing competition for maritime superiority as China and Russia build up their navies.
China's island development in the South China Sea has inflamed regional tensions, including with nations that have competing claims to the land formations.
Most fear that Beijing, which has built airfields and placed weapons systems on the man-made islands, will use the construction to extend its military reach and perhaps try to restrict navigation.
Three times in the past seven months, US warships deliberately have sailed close to one of those islands to exercise freedom of navigation and challenge the claims.
In response, China has deployed fighter jets and ships to track and warn off the American ships, and accused the US of provocative action.
In other places around the globe, including portions of India or large swaths of the South American coast, US ships routinely sail within claimed territorial waters or refuse to provide advance requests for transit. Often the operations go unnoticed or trigger no reaction or protest.
