When people picture diplomacy, they imagine ministers shaking hands, ambassadors presenting credentials and communiqués issued after summits. As U.S. ambassador to Portugal, I took part in all of that.

But I learned that the partnerships that last are often built below the national level, in city halls, regional councils, university labs and boardrooms and by local leaders who understand what a new investment means for the people who will work there.

I saw this most clearly when American technology companies, including the hyper-scalers building data centers that power the digital economy, made major investments in Portugal. National agreements set the frame. The projects succeeded because of sustained, practical engagement with mayors and regional leaders alongside the national government.

That work was rarely glamorous. It meant cutting red tape so workers had housing, connecting companies to nearby communities and helping residents understand the jobs that these investments would bring. No embassy can do that alone.

The same was true in reverse. When Portuguese companies looked to invest in the U.S., the questions included which state, city or region offered the best geographic, technical and regulatory fit. Answering those questions required relationships well beyond Washington, D.C.

This is subnational diplomacy in action. It has moved from the margins of foreign policy to its center. Foreign governments routinely send delegations to Phoenix and Kansas City, not just Washington, because governors and mayors help decide where companies invest and jobs are created.

Phoenix is a great example of what this looks like. Mayor Kate Gallego (D) told a seminar I led for the Meridian International Center that a major semiconductor investment in her city “would not have happened without those subnational relationships,” which helped Phoenix “get in the door and make the pitch for what is for our city a generational investment.”Local leaders are well suited to this work. A mayor understands how trade, investment and technology can become a live opportunity on a specific street. A governor knows which training programs can scale. The best local leaders know they cannot deliver at home without engaging abroad.

Marvin Rees, the former mayor of Bristol, England, who now sits in the House of Lords, told the Meridian audience that he could not deliver for his city without first working to shape the national and international context in which it operated.

The lesson I took from Lisbon is that relationships come first. Much of my job was bringing people from technology, business, government and civil society together to build trust and solve problems. Trust built across a table rarely shows up in a treaty. But it is what keeps a project moving when a permit stalls or a company needs a partner who will pick up the phone.

Subnational diplomacy is too often treated as an afterthought, a matter of sister-city ceremonies and trade-mission photo ops. National governments should treat it as a strategic asset. That means bringing mayors and governors into economic and technology policy conversations earlier and more often. It also means embassies doing more to connect foreign partners with American cities and towns beyond the coasts and the capital.

In a period of rapid technological change, America’s states, cities and towns are engines of ingenuity. As Kansas City Mayor Quinton Lucas told the seminar, subnational diplomacy is “an opportunity for America to really exist and operate at its best.”Some of the most consequential diplomacy of the coming decade will not happen at a summit. It will happen when mayors, executives and foreign officials sit down at the same table and decide to build something together. The job of American diplomats is to make sure that table is set.

Randi Charno Levine is a former U.S. ambassador to Portugal, a distinguished fellow in the Transatlantic Security Initiative within the Atlantic Council’s Scowcroft Center for Strategy and Security, and a trustee of Meridian International Center.