Now that North Korean president Kim Jong Un has rejected Donald Trump’s offer to “go big” and agree to immediate, full denuclearisation, the US president has a choice to make: double down or embrace the art of the possible. Despite the overtures, Mr Kim remains convinced that nuclear weapons guarantee his security, stature and survival. Full denuclearisation remains an important aspiration, but the more practical challenge is to reduce nuclear danger now.

As South Korean president Moon Jae-in visits the US this week, the experience of the Iran nuclear agreement — which Mr Trump declared the “worst deal ever” and abandoned — offers a path forward. Diplomacy is often a slog, focused on managing problems, not solving them. When successes come, they are usually at the margins, not in dramatic breakthroughs, and never purely the product of personal relationships between leaders.

That is certainly the lesson of nuclear diplomacy with Iran. As international concerns about Tehran’s nuclear programme mounted over 20 years, it was tempting for some to contemplate a grand bargain addressing the whole, poisonous range of US-Iran differences. Others were tempted by notions of regime change. Neither was realistic.