Every era of history is defined by its signature challenge. For the first half of the 20th century, it was what to do about German and Japanese militarism; for the second half, it was the struggle against the Soviet Union.

But today and for the foreseeable future, the principal threat to world order is not from some aggressive great power. Instead, we must contend with a host of global phenomena: the spread of nuclear materials and weapons, terrorism, pandemic disease, climate change and economic protectionism.

No country, not even the US, can face these challenges alone. The world is simply too large and too complex to control. By their nature, these challenges are best met by collective effort. Decisions to opt out of global arrangements (or an inability to opt in, as we see in the case of governments too weak to combat terrorists who set up shop on their territory) can have repercussions far beyond a country's borders.