In a wide-ranging speech on foreign policy to US military cadets at West Point, Obama said that the United States should shun isolationism and that its military must be prepared for crises.
"Regional aggression that goes unchecked - whether it's southern Ukraine, or the South China Sea, or anywhere else in the world - will ultimately impact our allies, and could draw in our military," Obama said.
But Obama emphasised caution on any decision to use force and said: "American influence is always stronger when we lead by example."
"We can't try to resolve the problems in the South China Sea when we have refused to make sure that the Law of the Sea Convention is ratified by the United States - despite the fact that our top military leaders say that the treaty advances our national security," Obama said, not naming China directly as he diverted from his prepared text.
"That's not leadership; that's retreat. That's not strength; that's weakness," Obama said.
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