A diplomatic bag, also known as a diplomatic pouch, is a container with certain legal protections used for carrying official correspondence or other items between a diplomatic mission and its home government or other diplomatic, consular, or otherwise official entities. The physical concept of a "diplomatic bag" is flexible and therefore can take many forms (e.g., a cardboard box, briefcase, duffel bag, large suitcase, crate or even a shipping container). Additionally, a diplomatic bag usually has some form of lock and/or tamper-evident seal attached to it in order to deter interference by unauthorized third parties. The most important point is that as long as it is externally marked to show its status, the "bag" has diplomatic immunity from search or seizure, as codified in article 27 of the 1961 Vienna Convention on Diplomatic Relations. It may only contain articles intended for official use. It is often escorted by a diplomatic courier, who is similarly immune from arrest and detention.
In contemporary practice, diplomatic bags are indeed used for exactly this purpose. An illustration is the strenuous protest made by German diplomats in Poland in the late 1920s when a cypher machine being shipped to the German Embassy in Warsaw – a commercial version of the famous Enigma machine – was mistakenly not marked as protected baggage and was opened, under protest, by Polish Customs. It was released to them, supposedly without much apology (and with still more protest), on the following Monday, but had perhaps been subjected to inspection by Polish cryptography personnel over the weekend.
Triplex was a British espionage operation in World War II which involved secretly copying the contents of diplomatic pouches of neutral countries.
In 1964, a Moroccan-born Israeli double agent named Mordechai Ben Masoud Louk (also known as Josef Dahan) was drugged, bound, and placed in a diplomatic bag at the Egyptian Embassy in Rome, but was rescued by Italian authorities. The crate that he had been placed in appeared to have been used for a similar purpose before, possibly for an Egyptian military official who had defected to Italy several years before but then disappeared without a trace before reappearing under Egyptian custody and facing trial.
During the 1982 Falklands War, the Argentine government used a diplomatic bag to smuggle several limpet mines to their embassy in Spain, to be used in the covert Operation Algeciras, in which Argentine agents were to blow up a British warship in the Royal Navy Dockyard at Gibraltar. The plot was uncovered and stopped by the Spanish police before the explosives could be set.
In the 1984 Dikko Affair, a former Nigerian government minister, Umaru Dikko, was kidnapped and placed in a shipping crate in an attempt to transport him from the United Kingdom back to Nigeria for trial. However, it was not marked as a diplomatic bag, which allowed British customs to open it.
In March 2000 Zimbabwe was the object of political interest internationally when it opened a British diplomatic shipment.
In May 2008, a replacement pump for the toilet on the International Space Station was sent in a diplomatic pouch from Russia to the United States in order to arrive before liftoff of the next shuttle mission.
In 2012, a 16 kg shipment of cocaine was sent to the United Nations in New York in a bag masquerading as a diplomatic pouch.
In January 2012, Italy detected 40 kilograms of cocaine smuggled in a diplomatic pouch from Ecuador, arresting five. Ecuador insisted it had inspected the shipment for drugs at the foreign ministry before it was sent to Milan.
In November 2013, the UK government alleged that a British diplomatic bag had been opened by the Guardia Civil at the Gibraltar-Spanish border, sparking a formal diplomatic protest. The Spanish government responded that the bag, being transported from the Governor of Gibraltar by a courier company within a mailbag containing other packages, did not meet the criteria of being in transit between a diplomatic mission and a home government.