As the 16th Supreme Allied Commander at NATO, my operational headquarters was in central Belgium, not far from the World War One battlefield at Verdun. During my time in command, I would occasionally walk through the fortifications of Verdun, something anyone can do today, exactly a century after the fierce battle that cost nearly a million casualties over months of fighting. As I walked through the long entrenchments, which were echoed across central Europe, I would ask myself, as a security professional, what went wrong? Why did so many have to die in that war and even more in the Second World War?

As I look back on the calamitous 20th century, in which the world endured two epic global conflicts with perhaps 100 million military and civilian dead (we’ll never know the real numbers), as well as a global depression and a long twilight Cold War, it occurs to me that much of our failures in the dimension of international relations and global security were the result of our propensity to build walls. In this turbulent 21st century, it seems to me walls are very seldom the answer. Instead, we have a better shot at creating lasting security by building bridges.

Let’s begin by admitting that occasionally walls make sense in tactical ways. We need tactical walls around secure compounds and bases, there are times when a temporary wall can separate deeply opposed factions, and the symbol of a wall can have political meaning, a la the 48th parallel in Korea. Barricades around various protected enclaves in Baghdad and Kabul have reduced suicide bombings. Israel has achieved tactical reductions in terrorist incidents through building walls.

But think about the vast strategic “walls” of the 20th century: the Hawley Smoot tariffs that were part of the global creation of tariff barriers in the run up to the Great Depression; the trenches of the First World War, overrun easily by the Schlieffen Plan; the failed Maginot Line of the Second World War; the Iron Curtin and its Asian cousin, the Bamboo Curtin; and most iconic of all, the Berlin Wall. How did all those walls work out?

They contributed significantly to two massive World Wars, the Great Depression, and lengthy Cold War. As a general proposition, walls don’t work. They fail tactically (you can go around them, over them, or through them); they fail operationally (by constraining the thinking and imagination of defenders and instilling a false sense of confidence); and they fail strategically (by limiting the opportunities for dialog, negotiation, settlement of disputes, collaboration, and comparative economic advantage).

Instead of thinking about how we can build bigger and better walls, we should be thinking about how we can build bridges, which will do more in the long term for our security than any other approach.

We should begin internationally with a foreign policy that seeks to connect us with allies (through formal treaties), partners (through personnel exchanges, operational exercises, and dialog), and even with our opponents (to modulate military encounters, maintain transparency about what is really happening, and understanding the motivations of others). Even with Russia, with whom we have significant disagreements, we have executed agreements to make sure our ships and planes don’t collide in an inadvertent incidents.

Another important level of bridge building is within our own government. Too often, we encounter gridlock and mistrust, often built on partisan bickering and an inability to put the nation’s interests above individual or party concerns – thus the branches of our government tend to grind to a halt. Even within the branches of government, there is too little cooperation, collaboration, and bridge-building and too much unwarranted competition and backbiting. In the domain of international affairs, ensuring seamless cooperation between the Department of Defense, the Department of State, the US Agency for International Development, and other key actors is crucial; yet you can still feel the walls within the executive branch quite plainly in Washington today.

Perhaps the most important set of bridges we need are between the government (which is not always the enemy of innovation and free-market solutions, but certainly creates a fair amount of drag) and the private sector. Of late, there are encouraging signs of cooperation in a variety of venues, including biotech, cyber security, information sharing, and maritime affairs – but still too little. The Department of Defense has opened an outpost in Silicon Valley and is reaching out for innovation to the private sector through the Defense Innovation Unit Experimental (DIUX).

All of this must be undergirded by a culture of strategic communication. The greatest bridge ever constructed is Facebook, which today connects nearly 1.6 billion people around the world. Along with the other social networks, Facebook represents a kind of crude, nascent post-Westphalian system of individual international organization. Will social networks overtake nation states? Probably not anytime soon, but it does present a vibrant opportunity to connect and build bridges between widely disparate populations. It is no surprise that the nations most interested in talking about segmenting the internet (essentially creating walls in cyber space) are repressive regimes like North Korea and Russia.

A final example: these days we hear a lot of loose talk about building a massive wall between the U.S. and Mexico. Here’s a news flash: on the left of any wall you build, however high and strong it may be (and expensive, regardless of who is paying) there is a vast Pacific Ocean that stretches for thousands of miles. On the right of this putative wall are the benign waters of the Gulf of Mexico. You don’t have to be an Admiral to predict that those who want to come badly enough will figure out a way to go around the wall via the sea. A better approach would be to work with our partners and friends in Mexico to enlist their cooperation in securing the shared border, address the conditions both in Mexico and more importantly in Central America that drive the migration, and develop a better and more coherent overall strategic plan than throwing up a wall.

As Ronald Reagan famously said to the Russians decades ago during a speech in Berlin, “tear down this wall.” That is pretty good advice generally in this turbulent 21st century.