India, July 7 -- US President Donald Trump departed for Ankara on Monday to join NATO leaders at what officials and analysts describe as one of the alliance's most consequential summits in years, not because dramatic breakthroughs are expected, but because the two-day gathering may determine what the transatlantic alliance looks like after decades of overwhelming American military leadership.

The July 7-8 summit unfolds as Washington openly prepares European allies for a gradual shift in U.S. military posture on the continent, presses members to sharply increase defence spending, and signals that Europe's security must increasingly become Europe's own responsibility.

Trump arrives carrying familiar grievances that many allies continue b...