President Trump will press NATO allies to fulfill vital defense spending commitments at a summit in Turkey this week.
The pressure follows assurances secured from member nations last year.
NATO's chief has demanded that allies submit robust plans to reach spending benchmarks.
U.S. officials have described heightening tensions as "growing pains."
Observers show allies will receive "report cards" to track their progress.
Various experts have alerted of financial consequences if the demands are enforced fully.