BRUSSELS — If Europe came into conflict with Russia, only several thousand of the more than 1 million troops in its armies would be ready for rapid deployment, military planners fear.

The U.S. now wants to step up readiness and ensure that at least 30,000 troops, plus additional aircraft and naval ships, can reach a trouble spot within 30 days of NATO commanders putting forces on alert, current and former allied officials say. Tensions between Russia and the West are as high as they have been in years.