Byzantine diplomacy in the context of Biblioteca Nacional de España

The Byzantine–Bulgarian war of 913–927 (Bulgarian: Българо–византийска война от 913–927) was fought between the Bulgarian Empire and the Byzantine Empire for more than a decade. Although the war was provoked by the Byzantine emperor Alexander's decision to discontinue paying an annual tribute to Bulgaria, the military and ideological initiative was held by Simeon I of Bulgaria, who demanded to be recognized as Tsar and made it clear that he aimed to conquer not only Constantinople but the rest of the Byzantine Empire, as well.

In 917, the Bulgarian army dealt a crushing defeat to the Byzantines at the Battle of Achelous, resulting in Bulgaria's total military supremacy in the Balkans. The Bulgarians again defeated the Byzantines at Katasyrtai in 917, Pegae in 921 and Constantinople in 922. The Bulgarians also captured the important city of Adrianople in Thrace and seized the capital of the Theme of Hellas, Thebes, deep in southern Greece. Following the disaster at Achelous, Byzantine diplomacy incited the Principality of Serbia to attack Bulgaria from the west, but this assault was easily contained. In 924, the Serbs ambushed and defeated a small Bulgarian army on its way to Serbia, provoking a major retaliatory campaign that ended with Bulgaria's annexation of Serbia at the end of that year.

The Bulgarian–Serbian wars of 917–924 (Bulgarian: Българо–сръбски войни от 917–924) were a series of conflicts fought between the Bulgarian Empire and the Principality of Serbia as a part of the greater Byzantine–Bulgarian war of 913–927. After the Byzantine army was annihilated by the Bulgarians in the battle of Achelous, the Byzantine diplomacy incited the Principality of Serbia to attack Bulgaria from the west. The Bulgarians dealt with that threat and replaced the Serbian prince with a protégé of their own. In the following years the two empires competed for control over Serbia. In 924 the Serbs rose again, ambushed and defeated a small Bulgarian army. That turn of events provoked a major retaliatory campaign that ended with the annexation of Serbia in the end of the same year.

An apocrisiarius, the Latinized form of apokrisiarios (Greek: ἀποκρισιάριος), sometimes Anglicized as apocrisiary, was a high diplomatic representative during Late Antiquity and the early Middle Ages. The corresponding (purist) Latin term was responsalis ("he who answers"). The title was used by Byzantine ambassadors, as well as by the representatives of bishops to the secular authorities. The closest modern equivalent is a papal nuncio; the title apocrisiarius is also still employed by the Anglican Church.

Between the Roman Empire and the Han dynasty, as well as between the later Eastern Roman Empire and various successive Chinese dynasties, there were (primarily indirect) contacts and flows of trade goods, information, and occasional travelers. These empires inched progressively closer to each other in the course of the Roman expansion into ancient Western Asia and of the simultaneous Han military incursions into Central Asia. Mutual awareness remained low, and firm knowledge about each other was limited. Surviving records document only a few attempts at direct contact. Intermediate empires such as the Parthians and Kushans, seeking to maintain control over the lucrative silk trade, inhibited direct contact between the two ancient Eurasian powers. In 97 AD, the Chinese general Ban Chao tried to send his envoy Gan Ying to Rome, but Parthians dissuaded Gan from venturing beyond the Persian Gulf. Ancient Chinese historians recorded several alleged Roman emissaries to China. The first one on record, supposedly either from the Roman emperor Antoninus Pius or from his adopted son Marcus Aurelius, arrived in 166 AD. Others are recorded as arriving in 226 and 284 AD, followed by a long hiatus until the first recorded Byzantine embassy in 643 AD.

The indirect exchange of goods on land along the Silk Road and sea routes involved (for example) Chinese silk, Roman glassware and high-quality cloth. Roman coins minted from the 1st century AD onwards have been found in China, as well as a coin of Maximian (Roman emperor from 286 to 305 AD) and medallions from the reigns of Antoninus Pius (r. 138–161 AD) and Marcus Aurelius (r. 161–180 AD) in Jiaozhi (in present-day Vietnam), the same region at which Chinese sources claim the Romans first landed. Roman glassware and silverware have been discovered at Chinese archaeological sites dated to the Han period (202 BC to 220 AD). Roman coins and glass beads have also been found in the Japanese archipelago.