If a visitor who knew nothing about geopolitics came to Washington, would he see the dazzling capital of the most powerful nation in the world, as Americans like to think we are and always will be? Or might he conclude, looking at our public works, that this nation’s best days ended around the time of President Kennedy?

Take Dulles International Airport, our gateway for many international visitors. It boasts a beautiful, architecturally renowned terminal — that was built in 1956. Almost every part of the facility newer than that seems shoehorned in, such as the escalators from the A gates to the new AirTrain or the long underground walkway to the now-29-year-old “temporary” C gates. We are building a Metro line to the airport, but it will inconveniently deposit passengers in a parking garage 600 feet from the terminal.

Union Station, long the front door to the capital, is an even more breathtaking structure. Our nation once let it fall into disrepair, then finally fixed it up, mainly to create a shopping mall. Today, the experience for anyone getting off a train into Washington or, worse yet, trying to board one involves massive jams just to get out of the connected Metro station and grossly overcrowded waiting areas that seem almost an afterthought.

For a while, the Metro system was our region’s pride and joy — until we failed to maintain it for 30 years. Now, escalator failures, bent tracks, hot cars and weekend shutdowns for repairs make it a constant source of complaints on Twitter and other social media. Too few of the complaints, however, point the finger at the state, District and federal budgeters who created the problem.

When did Americans stop believing in having a transportation infrastructure that befits a great nation? China, Brazil, and India are building dazzling train stations and subways, high-speed rail systems, impressive airports and even new cities. What about us?

Infrastructure like this is about far more than pride; it’s also about economics. Metro saves households an estimated $1.1 billion per year in time and in car expenses, while the areas around Metro stations have gained $212 billion in value since the system was built. The system has dramatically reduced costs for the federal government and its workers (even though the federal government hardly chips in to keep it running).

We have opportunities to again invest in our future and keep the economic growth coming. For example, to accommodate riders of Amtrak, MARC, VRE, Metro, and local and intercity buses, these transportation agencies have collaborated on a bold and much-needed master plan that would turn the crowded gateway to our capital into a fitting and modern terminal with enough track and platform capacity for the next 100 years.

Behind the historic hall, which would remain, a new, glass-enclosed structure would connect to concourses that would serve Maryland and Virginia’s rapidly growing commuter railroads and move passengers directly to the new office district of NoMa and the burgeoning neighborhoods nearby. It’s a vision for what would be a 15- to 20-year project, and it’s not cheap. But we need to dream big, and long term, the way the generations who built today’s Washington did.

Metro, too, has been working on long-term plans to serve growing neighborhoods and relieve the bottlenecks that limit how many trains can cross the Potomac River. Montgomery County has its Purple Line and Bus Rapid Transit plans, which would transform transportation in the county, if it and the state can fund them.

With all of these plans come the fretting about the price tag. Can we afford to build this infrastructure? The real question is, can we afford not to? Has our nation become so complacent that we will no longer invest in our future? We can’t remain the epicenter of the world economy while our infrastructure, in our capital and elsewhere, crumbles.

Deputy U.S. Transportation Secretary John Porcari, Maryland’s former transportation chief, had this to say at the news conference unveiling the Union Station plan:

“You can look at the station, you can look at infrastructure here in Washington or anywhere else in the country. Look at it closely. And if you’re honest with yourselves, you know that it was designed and built and paid for by our parents and grandparents, and in some cases by our great-grandparents. Then you have to ask yourself the next question: Are we doing right by the next generation? Are we paying it forward? Do we have a bold vision that will make a foundational investment in economic development that transportation is? Well, I think you know the answer to that. As of today, we’re not.”

Our hypothetical visitor might well assume we are a nation that dreamed big and took pride in its cities, including its capital, and then retreated into a shell a generation ago. Sadly, this visitor would probably be right. We can dream again, and if we want to be great again and remain so, we must.

The writer is the editor of the blog Greater Greater Washington. He participates in The Post’s Local Blog Network.