"America will never be destroyed from the outside," Abraham Lincoln once said. "If we falter and lose our freedoms, it will be because we destroyed ourselves."

There's more than one way to inflict such fatal harm. It doesn't have to happen overnight, or even by design. It can occur slowly, over time — through simple neglect.

Consider what's been happening with our military. No one set out to degrade our readiness. But 17 years of a long war against terrorism, combined with shrinking budgets, have taken their toll. Our brave troops work hard to do all that we ask of them, but they've been stretched to their limits, making do with aging ships, planes and tanks.

But not our adversaries. While the U.S. rested on the investments of past administrations, they've capitalized on the growing availability of advanced technologies. They've been drawing from global commercial innovation, stealing the intellectual property of American businesses, and developing ways to counter long-held U.S. advantages in every domain of warfare.

The threats we face have grown increasingly sophisticated. According to Secretary of Defense Jim Mattis, "We cannot expect success fighting tomorrow's conflicts with yesterday's weapons and equipment."

The Big Problem: Complacency

Yet the alarm bells haven't been sounding loud enough. U.S. military superiority has bred complacency among a population that has never known military defeat. As long as our country largely takes for granted the peace and prosperity won through generations of investment and sacrifice, we risk learning the hard way that continued superiority is not assured.

Retaining a military advantage — particularly under the current pace of technological development — requires an enduring commitment to consistent investment in our country's security. Our history shows that a strong, capable military deters aggression and effectively enhances our ability to engage the world through diplomacy and trade.

"Speak softly and carry a big stick," President Theodore Roosevelt famously said. Well, that works only if you ensure that the stick remains big — and at the ready.

Fortunately, Congress recently took a step toward reversing the underfunding that has long plagued our military. The Bipartisan Budget Act of 2018 provided two years of welcome relief from the threat of sequestration.

Long-Term Damage

But there is much more to do. Rebounding budgets and returning strength point to positive trends for U.S. national defense. However, the damage done over many years will not be undone overnight.

Congress and the president must stay the course. Decades of continuing resolutions and budgetary uncertainty have left the military hostage to political whims, unable to plan and prepare for challenges on the horizon.

Consider these alarming highlights from the just-released "2019 Index of U.S. Military Strength":

• The Air Force and Marine Corps both received "weak" readiness ratings, with the Air Force mired in a crippling shortage of fighter pilots and fighter aircraft. The average fighter pilot is currently flying fewer than two times a week, severely degrading combat readiness of the force.
• Of the U.S. Army's 31 Brigade Combat Teams (BCTs), the building blocks of American ground combat power, only 15 are considered "ready," and only eight of those are "fully ready." Army leaders have said it could be 2022 before the service reaches its goal of having two-thirds of active BCTs ready.
• The Army, Navy and Air Force all received overall ratings of "marginal" from Index editors, but the Marine Corps has an overall rating of "weak," with approximately half of its amphibious ship and tactical aircraft fleets unavailable for current operations.

Our military needs more than a temporary jolt of extra funding. As the world returns to an era of great-power competition, it is not enough simply to repair and replace aging ships, planes and tanks. We need to rebuild on a scale not witnessed since the 1980s.

Military Must Remain Strong

Some lawmakers contend that this is impossible. Indeed, the greatest threat to U.S. military strength is the misconception that America can no longer afford military superiority.

Yes, it will require some hard choices. Entitlement costs consume an increasing portion of the federal budget, and Congress continues to blow through debt ceilings. But we're talking about our national security. Too many treat defense as a trade-off rather than as the obligation and responsibility it truly is: a constitutionally mandated function of government to provide for the common defense.

The 2019 Index of U.S. Military Strength provides an enduring benchmark for Congress based on what history has shown is necessary to defend national interests. The force we need will not come cheap, but the costs of weakness and complacency are far greater.