What happened in 740? — events across the world

West Asia

Battle of Akroinon
740
In 740, the Byzantine Empire decisively defeated an Umayyad Arab army at Akroinon in Phrygia. The Arab raids into Anatolia had been ongoing for over a century, and the 740 expedition was the largest in recent decades. Emperor Leo III and his son Constantine V confronted a force of about 20,000 under Abdallah al-Battal. The Byzantine victory, combined with Umayyad troubles on other fronts and internal instability, ended major Arab incursions into Anatolia for three decades.
SourcesWikipedia ↗Image: Bigdaddy1204 at English Wikipedia / CC BY-SA 3.0 ↗
Arab conquest of Armenia
642–744Greater Armenia
The Arab conquest of Armenia was a series of military campaigns launched in the mid-7th century by the Islamic polity of the Arabian Peninsula against the Byzantine-controlled region of Armenia, as part of the early Muslim conquests and the Arab–Byzantine wars.
SourcesWikipedia ↗Image: Original: Mohammad Adil ( talk ) Derivative: باسم / CC BY-SA 3.0 ↗
Zayd ibn Ali's Revolt
740Kufa
In 740, Zayd ibn Ali, great-grandson of Ali, rose in revolt against the Umayyad Caliphate in Kufa. The uprising failed swiftly and Zayd was killed, but his son Yahya continued the struggle, marking a foundational event for Zaydi Shiism.
SourcesWikipedia ↗

Founding of the Tang Dynasty
618–907Chang'an (Tang capital site), Xi'an, Shaanxi, China
The Tang dynasty was founded by Li Yuan in 618, replacing the Sui dynasty. It became a major unified dynasty in Chinese history, with its capital at Chang'an.
SourcesWikipedia ↗Image: Ian Kiu / CC BY-SA 3.0 ↗
Taika Reform
645–894Naniwa-kyō (Naniwa Palace site), Osaka, Japan
In 645, Emperor Kōtoku of Japan enacted the Taika Reforms, aiming to abolish clan rule and establish a centralized state modeled on Tang China, profoundly impacting Japanese history.
SourcesWikipedia ↗

740 Constantinople earthquake
740
On 26 October 740, an earthquake struck in the vicinity of Constantinople and the Sea of Marmara.
SourcesWikipedia ↗

Berber Revolt (740–743)
739–743al-Andalus
Between 740 and 743, during the reign of the Umayyad caliph Hisham ibn Abd al-Malik, a major Berber revolt erupted. Inspired by Kharijite preachers, the Berbers rose against heavy taxes and Arab rule, led initially by Maysara al-Matghari in Tangier. The revolt spread to the rest of the Maghreb and crossed the strait to al-Andalus.
SourcesWikipedia ↗

Outbreak of the Third Tikal–Calakmul War
720–744Guatemala City
The third major war erupted between Tikal and Calakmul, two superpowers of the Classic Maya period, reflecting the protracted struggle for hegemony.
SourcesWikipedia ↗Image: Dennis G. Jarvis / CC BY-SA 2.0 ↗

Muslim conquest of Transoxiana
673–751Transoxiana
In the 7th and 8th centuries, Arab armies of the Umayyad and Abbasid caliphates conquered Transoxiana in Central Asia. The region lies between the Syr Darya and Amu Darya rivers, covering parts of modern Uzbekistan, Tajikistan, Kazakhstan, and Kyrgyzstan.
SourcesWikipedia ↗Image: Dr. Husein Mu'nis et al. / Public domain ↗

These are known only to the century, not to the year. They are listed separately so a rough date is never mistaken for an exact one.