Universal passport requirements are roughly a century old, and for a long period before that international travel required no documents at all.
The earlier documents
Letters requesting safe passage for a named individual existed for centuries.
Which were requests rather than requirements, issued to specific travellers, and they did not constitute a system.
Most people never travelled far enough to need one.
The open period
Through much of the nineteenth century, passport requirements were relaxed or abolished in many countries.
Which reflected a period of comparatively free movement, and it coincided with substantial migration.
Travel across many borders required no documentation whatsoever.
The change
The First World War produced security concerns that led to reintroduction of controls.
Which was intended as temporary and became permanent.
The combination of security concern, the ability to identify individuals photographically, and administrative capacity made systematic control feasible for the first time.
Standardisation
International conferences in the early twentieth century agreed common formats.
Which specified size, layout, languages and content, allowing officials to read documents issued elsewhere.
Later standardisation added machine-readable zones and, more recently, electronic chips containing biometric data.
These are set by an international civil aviation body, which is why passports worldwide share a format.
The refugee document
Displaced people without state documentation could not travel at all under the new system.
Which produced an identity document issued by an international body for stateless people.
It established the principle that documentation could exist independently of citizenship, which underlies subsequent refugee travel arrangements.
Visas
Permission to enter, distinct from the passport establishing identity and nationality.
Which developed alongside and has become the primary mechanism of migration control.
Visa requirements are generally reciprocal in principle and are frequently asymmetric in practice, reflecting relative negotiating positions.
The mobility gap
Passport holders from different countries face enormously different visa requirements.
Which is measured by indices ranking passports by the number of destinations accessible without prior visa.
The range between the most and least mobile is very wide, and it correlates with wealth and with geopolitical position.
Biometrics and automation
Electronic passports containing facial and fingerprint data enable automated border control.
Which speeds processing and raises questions about data retention, accuracy across demographic groups and function creep.
Automated systems have shown differential error rates across groups in testing, which has attracted regulatory attention.
Identity documents generally
National identity cards exist in many countries and not in others, reflecting different political traditions.
Which affects how passports function, since in countries without identity cards the passport becomes the default proof of identity.
Proposals for identity cards have been repeatedly contested in countries without them, on privacy and civil liberties grounds.
Borders and technology
Advance passenger information, electronic travel authorisations and biometric matching have shifted checking earlier in the journey.
Which means decisions about entry are increasingly made before travel rather than at the border.
Carrier liability, holding airlines responsible for passengers without proper documents, effectively delegates checking to private companies.
Statelessness
People without any nationality face substantial barriers, since documentation depends on a state issuing it.
Which affects millions of people according to international estimates, and conventions addressing it exist with limited ratification.
Birth registration is the practical foundation, and substantial numbers of births go unregistered globally.
Free movement areas
Arrangements removing internal border checks between participating states.
Which exist in several regions with different scope, and they demonstrate that the current system is a policy choice rather than a necessity.
Consular protection
The passport also functions as a request for assistance from the issuing state's representatives abroad.
Which is the original function preserved in the wording printed inside most passports.
What consular assistance actually covers is narrower than most travellers assume and is published by foreign ministries.
Dual nationality
Permitted by some states and prohibited by others, with consequences for military service, taxation and consular protection.
Which produces complications where two states both claim a person as a national.
Some states do not recognise the other nationality of their own citizens, which limits consular access.
Travel advice
Foreign ministries publish country-specific guidance affecting insurance validity and, in some cases, legal obligations.
Which means travel against official advice can invalidate insurance, and this is frequently discovered after a claim.
Digital identity
Proposals for digital travel credentials stored on devices are being trialled.
Which would extend the trajectory from paper to machine-readable to biometric to digital.
Standards work is under way through the same international body that standardised the physical document.
Border checks and their cost
Processing capacity at borders determines waiting times, and staffing decisions are effectively policy decisions.
Which has produced substantial delays at several borders following changes in requirements.
Automated gates have increased throughput where deployed and require eligible document types.
Which means a document type determines which lane you can use, and older documents are excluded from the faster route.
Validity requirements for onward travel differ by destination and catch travellers regularly.