Since the early days of our republic, America’s military has won tough victories by pursuing innovative tactical advantages and challenging the unwritten rules of how “modern” wars should be fought. In the American Revolution, while the British were standing in lines banging on drums in open fields, America’s ragtag guerilla Patriots were in the woods quietly waiting to topple the world’s most powerful army. In WWII, Eisenhower led the Allied attack on Normandy and secured victory on the European front after a strategic pump-fake fooled Axis military leaders and left an opening for our brave troops. From game-changing technologies like airplanes and satellites to inventive battlefield and intelligence strategies, our nation has always succeeded by fighting wars on our terms.

With the release of the president’s budget this week, Washington is once again debating the investments we must make to keep our military strong and the American people safe. The stakes of this debate could not be higher. Decisions that Congress makes today will determine how prepared the United States is to face our gravest threats today and for decades to come.

But we must resist the tendency to, as the saying goes, fight the last war. Weapons programs that made sense yesterday may no longer be the wisest investment for scarce taxpayer dollars, and we have to be willing to make the tough decisions that will prepare our military for the threats we are likely to face.

In some ways, today’s military is still grounded in a WWII or Cold War mentality, where massive armies and hundreds of thousands of ground troops held territory in a clash of great powers. The next wars will look different. The threats we must prepare for are more unpredictable, messier, and on a smaller scale, with non-state actors like ISIL as well as competitors like Russia and China challenging us in arenas far from traditional battlefields.  And while we must prepare to defend against both conventional and unconventional threats, the tools that matter most will be cyber, advanced standoff weaponry, agility, and stealth.

That is why the tradeoffs and investments—particularly in things like state-of-the-art submarines and advanced aircraft—proposed in this week’s budget rollout are so critical. I stand firmly with Secretary Carter in his call to right-size our Army, reducing numbers of active duty soldiers and investing more in the advanced weapons that will make us more agile and effective.  The budget recognizes that some legacy investments – like tactical aircraft – need to be expanded due to the heavy demand for U.S. air power in the fight against terrorism.  And while the number of nuclear weapons can certainly be curtailed, the safest way to deploy our nuclear deterrent is on the next generation of ballistic missile submarines, a key investment in this new budget. 

Mobility, stealth, and advanced technology—not simply thousands of troops stationed overseas or an endless supply of land based nuclear weapons—are what will enable America to maintain superiority in the many unpredictable challenges we face. Whether it’s next-generation submarines in the Pacific, air superiority and reconnaissance over Eastern Europe, or precision munitions to destroy terrorists while limiting civilian casualties, it is technology and agility that are critical. Fortunately, our defense manufacturers are the best in the world, and are hard at work designing and building the next generation of subs and aircraft to maintain our competitive technological advantage. But we must not neglect these investments as budgets tighten.

In the last 30 years, our enemies have changed, the battlefields have morphed, and the way wars are fought has been transformed. That’s why we can’t let muscle memory dictate our investments in national defense. We have to be tough and we have to be smart. Congress needs to remember that if our Pentagon budget is written to win the last war, we won’t win the next one.

Murphy is Connecticut’s junior senator, serving since 2013. He sits on the Appropriations Committee; the Foreign Relations Committee; and the Health, Education, Labor and Pensions Committee.