Justifiable journeys or junket jamborees?

American presidents do not have the legal power to prevent members of the US Congress travelling abroad, but they can stop them getting taxpayer-funded flights on military planes.

In denying that privilege to Nancy Pelosi — his arch Democrat rival in the House of Representatives and a possible contender in the 2020 presidential election — for her trip to Afghanistan, Egypt, and Belgium, Mr Trump has given way to his taste for tit-for-tat retaliation.