President Trump inherited a military in crisis. The Obama administration asked a shrinking force, with aging equipment and declining budgets, to fight wars and perform other missions around the world. Gridlock on Capitol Hill added to the problem, delivering late and inadequate budgets for nearly a decade. The result was a readiness crisis with tragic, sometimes fatal consequences for American troops.

China and Russia, seeking to make the world safer for authoritarianism, have rapidly modernized their militaries. The U.S. advantage has eroded in key areas: power projection, cyberdefense, space, electronic warfare, air and missile defense, antisubmarine warfare, and long-range ground-based fires. America “might struggle to win, or perhaps lose, a war against China or Russia,” warns the bipartisan National Defense Strategy Commission in a report this month.

The administration’s National Defense Strategy prioritized strategic competition with China and Russia. But to be effective, strategies must be matched with resources. America won’t succeed without sustained, sufficient, predictable military funding.

The past two years have put our military on a better path. In March 2017 Mr. Trump requested $30 billion more in defense spending to begin repairing readiness. His first two budget requests focused on restoring America’s military advantage. Congress followed through this year. For the first time in a decade, we funded the troops in full and on time.

Next year the president and Congress face a critical national-security decision: Will we continue to rebuild our military, or will we squander our progress? Media reports indicate the administration is telling the Pentagon to plan for a $700 billion budget in fiscal 2020. That’s $16 billion less than 2019 and $33 billion less than Mr. Trump’s original budget plan.