As President Biden and his aides planned NATO's 75th anniversary, which opens Tuesday evening in Washington, the intent was to create an aura of trust.The message to Russian President Vladimir V. Putin and other potential adversaries would be that, after more than two years of war in Ukraine, a larger and more powerful group of Western allies has emerged, more determined than ever to repel aggression.But as 38 world leaders began arriving here on Monday, that confidence appears to be in jeopardy. Even before the summit formally began, it has been clouded by uncertainty over whether Mr Biden will remain in the running for a second term and the looming possibility of former President Donald J. Trump’s return.Mr. Trump once declared NATO “obsolete,” threatened to leave the alliance, and m...