The U.S. Navy carried out two high profile aircraft carrier training events in key waters that send messages to both China and Russia, the U.S.’s two main competitors and the only countries close to matching the U.S.’s military might.

The U.S. Navy’s Ronald Reagan Carrier Strike Group joined Japan’s Maritime Self-Defense Force’s Escort Flotilla 4 Battle Group and conducted joint military exercises in the hotly-contested South China Sea Friday, Navy said.

Japan sent the Kaga, a small aircraft carrier technically classified as a destroyer, along with guided-missile destroyers to train with the US’s only forward-deployed aircraft carrier, the Reagan.

This training advanced the U.S. and Japan’s vision of a “free and open Indo-Pacific,” a key part of U.S. strategy to prevent Beijing from tightening its grip on the region by further militarizing the South China Sea.

But beyond just teaching U.S. and Japanese carriers how to fight together, the U.S. sent Beijing a message that it won’t be pushed out of the South China Sea, and if a fight comes, it won’t stand alone.

China, which illegally annexed about 90% of the South China Sea and has sought to unilaterally dictate who can use the resource-rich waterway that sees trillions in annual trade, has struggled to make allies in the region. The U.S. has moved to counter China’s attempts at hegemony with deeper ties with Australia, Japan, and India.

On top of that, the U.S. just showed for the first time ever that it can update its supercarriers with stealth aircraft perfect for taking out island fortresses like Beijing’s South China Sea holdings: The F-35C.

Russia checked by the 2nd fleet

us navy russia chinaU.S. Navy/Mass Communication Specialist 1st Class Brian M. Brooks
Aircraft from the Freedom Fighters of Carrier Air Wing 7 fly in formation above the Nimitz-class aircraft carriers USS Abraham Lincoln and USS Harry S. Truman.

Half a world away, the USS Abraham Lincoln and USS Harry S. Truman carriers did joint training including the F-35C for the first time. But this drill likely had an additional audience in mind — Russia.

The U.S. recently decided to bring back the 2nd Fleet, a Navy command that countered the threat from the Soviet Union and was stood down in 2011 when it seemed like the Russia threat had waned.

As Russia’s navy increasingly menaces the U.S. and looks to assert itself as a powerful navy in the Mediterranean and elsewhere, the U.S. has again found the need to defend the home waters of the near Atlantic.

Russia, which only has one inactive and shoddy aircraft carrier, cannot hope to compete with the U.S.’s multiple carriers and advanced aircraft.

The U.S. has recently reshuffled its schedule of aircraft carrier deployments to have more ships present to keep the pressure on Russia and China. New U.S. national defense and strategy documents from President Donald Trump’s administration decidedly shift U.S. focus from a post-Cold War mentality when the U.S.’s enemies where small, lightly armed cells of terrorists hidden in hills to a full on competition between world powers, as it was in the World Wars.

Related: The Navy Is Already Implementing Mattis’ Unpredictable New Aircraft Carrier Strategy »

Russia and China have taken notice, with Russian ships drilling in the Mediterranean, waters they wouldn’t have normally reached before their incursion into Syria in 2015, and Chinese ships challenging U.S. ships and planes right to pass through international spaces.

Also in 2015, the US suspended freedom of navigation patrols, its main way of checking Chinese ambition in the South China Sea.

But now, the Navy is taking those challenges seriously. “We are the best Navy in the world, and given the complex and competitive environment we are in, we can’t take anything for granted or settle for the status quo,” said Abraham Lincoln Strike Group Commander Rear Adm. John Wade in a Navy release.

With a renewed mission and the world’s first carrier-launched stealth aircraft, the U.S. has sent a clear signal to its main military rivals that US Navy power is back and on the move.

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