Trump argued that individual nations should act in their own self-interest, yet rally together when faced with a common threat.
President addresses body for 1st time, pledges support.
Haley makes comments as UN Security Council members push for more sanctions.
Following U.S. warnings to North Korea of "massive military response," South Korea fired missiles into sea to simulate attack on North's main nuclear testing site.
President says country ‘signaled its contempt’ for neighbors, UN.