NATO's 5% spending commitment is a watershed moment for an alliance dogged by laggard investment outside the U.S. — if, that is, members actually fork over the trillions in long-term spending required to make it a reality.

Why it matters: At a time of weapons upheaval and global war, friends are precious.

• New blocs of influence are solidifying. China, Iran and North Korea fuel a Russian war machine in Eastern Europe while the U.S. reevaluates its role abroad, including arms deliveries.
• There's a newfound determination in many capitals to invest in European security and reduce reliance on Washington.
• The question is whether it's a fleeting feeling or the beginnings of a decade-long push to hit 5%.

The big picture: The spending outlook, according to defense analysts, industry executives and Europe watchers Axios consulted, is complicated by these four factors:

Proximity and fears of invasion.

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