Unity is the central goal of this year's NATO summit. Secretary-General Mark Rutte is working to keep the alliance focused on shared security challenges as tensions linger between the United States and its European allies.

Though NATO members have pledged to boost defense spending, concerns remain over Washington's long-term military commitment to Europe. Rutte is betting that closer defense-industrial cooperation and increased military investments can help hold the alliance together.

The summit will be a crucial test of NATO's ability to present a united, credible and capable front in defending its members.