Since human beings first spread themselves across the earth, they have encountered obstacles: rivers, mountains, scarcity of food, extreme heat, biting cold, warlike peoples and diseases – physical barriers to progress. Because of difficulties such as these, the vast majority of human history has been lived in fear.

Some individuals and civilisations embrace that fear and use it as a catalyst to attempt great feats. Ever since our species has sought new lands, one of the devices used to overcome obstacles to movement is the bridge. Bridges allowed us to eat today and build tomorrow while not drowning in the attempt. Julius Caesar made use of huge temporary bridges in order to subdue (or more often, massacre) Germanic tribes over 2,000 years ago. We use them to carry water and to drive further.

Retrospectively, however, bridges can carry more than water or cargo; they carry a lot of history. Of the three colossal countries that comprise continental North America, two of them – Canada and the United States – have capital cities that are located in parts of their respective countries in such a way that part of the cities almost became part of another country, and what links these would-be sister states are bridges. Ottawa and Washington, the respective capitals, were both capital cities by the time of the Quebec referendums and the US Civil War, which makes their capital status before these historic events all the more important and extraordinary.

When you walk, as I did two weeks ago, from Arlington National Cemetery across the Potomac River on Memorial Bridge to the Lincoln Memorial, you are walking from the state of Virginia to the District of Columbia. You are also walking from the old Confederate States of America to what was then and is now the United States of America; in another history, you could have been walking from one nation to another. The unity that Washington brings to such a vast country can be seen in concentrated form right there on that small plot of land. The bridges are stitches on an old wound.

Earlier this summer, I also walked from Parliament Hill in Ottawa, Ontario across the Alexandra Bridge over the Ottawa River to Gatineau, Quebec. Just as in Washington, bridges link what might have become (and might yet become) an entirely separate country within the capital city. I cannot think of any other cities in the world where this is the case.

Of course, the ways that Quebec and the Confederacy went about trying to gain independence were entirely different. For the former, two referendums ended with Quebec remaining within Canada – nobody died, at least not directly. In contrast, more than 600,000 people died during the American Civil War.

Washington is a far more grandiose city. It is larger and more monolithic with more tourists and bigger things. When one takes that walk from the old Confederacy to the modern American capital – from the South to the North – the air oozes the salient sense that this is a very important place. In that respect, the city is quite beautiful because it is a beautiful thing to be there. Physically and politically, Washington itself is a monument of monuments.

Ottawa’s beauty is more understated, built as much by the hand of nature as by the hand of humanity. Taking into account how recently the city was almost split between two countries (1980 and 1995), its status becomes a significant factor in the viability of Canada remaining one country.

Both Ottawa and Washington are metropolises that could have been permanently split by rivers, politics and war. As it is, they are held together by the will of the majority of the people and by bridges, bridges that have a modern resonance far beyond their original function.