Follow-up question. When did passports become mandatory for travel? My great-grandparents emigrated to the USA and returned to the UK without passports (1900s) and my grandfather went ashore in many countries as a merchant seaman without ever getting a passport (1910s-1940s).

Depends on what you mean. Today a passport is a document that shows your identity. Sometimes countries decide to be selective who they let in, so they may allow or refuse entry based on that identity. Thus you could say that a passport is a sort of a pass that allows entry.

The first travel documents which came into existance were of that second variety, it was a sort of a "letter of request". One such was issued around 450BC by the Persian King to an official for travel to Judah. The letter requested safe passage and was addressed to the governors of the province.

Those "passports" were mostly used to allow passage to a town or place. Today's Canadian passports for example still include such "letter of request", issued in the name of the head of state (Her Majesty the Queen), inside the front cover.

But it took ages for those "letters of request" to became popular. The French King Louis XIV started to grant these documents to court favourites in the 17th century. A new name appeared, from "pass" + "porte" since most international travel included sailing and going through ports. By the 18th century almost every country in Europe had set up a system to issue passports. Passports were issued by the home country but there were also "visas" which were issued by the destination country.

Sometimes it got really complicated. Up to 1862 Canadians could travel freely to the US but when going to Europe they needed to obtain a passport from London, since they were British subjects. With the start of the Civil War, the US requested passports for the Canadians as well. Those were actually "letters of request" signed by the Governor General and not real passports, although they functioned as such.

The earliest of today's type passport was a document issued in 16c England. Those could prove the person's identity when travelling abroad, and were issued by a designated authority - the Privy Council. In 1794, passports began to be granted by the Secretarty of State and records are kept of all passports issued ever since in the country. At first passports were only single sheet.

With the boom of rail travel in the 19th century though, the multitude of tourists caused a complete breakdown in the passport/visa system on the Continent. France gave up first and abolished travel documents in 1861, with others following its example.

Then, the World War of 1914 again returned the need for border security and passports were back as well, this time with an added photograph. There also used to be a description of persons features, shape of face, complexion, etc. In 1920 the "League of Nations" (later to be replaced by the UN) issued an agreement to standartise passports.

Further reads: "The Cambridge economic history of Europe" (Cambridge University Press)