President Trump will push 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 plausible plans to reach spending targets.
U.S. officials have described intensifying tensions as "growing pains."
Observers reveal allies will get "report cards" to track their progress.
Various experts have cautioned of fiscal consequences if the demands are applied fully.