Naval warfare
In the twilight of the Roman Empire in the late 4th century, examples include that of Emperor Majorian, who, with the help of Constantinople, mustered a large fleet in a failed effort to expel the Germanic invaders from their recently conquered African territories, and a defeat of an Ostrogothic fleet at Sena Gallica in the Adriatic Sea.
During the Muslim conquests of the 7th century, Muslim fleets first appeared, raiding Sicily in 652 (see History of Islam in southern Italy and Emirate of Sicily), and defeating the Byzantine Navy in 655. Constantinople was saved from a prolonged Arab siege in 678 by the invention of Greek fire, an early form of flamethrower that was devastating to the ships in the besieging fleet. These were the first of many encounters during the Byzantine-Arab Wars.
The Caliphate became the dominant naval power in the Mediterranean Sea from the 7th to 13th centuries, during what is known as the Islamic Golden Age. One of the most significant inventions in medieval naval warfare was the torpedo, invented in Syria by the Arab inventor Hasan al-Rammah in 1275. His torpedo ran on water with a rocket system filled with explosive gunpowder materials and had three firing points. It was an effective weapon against ships.
In the 8th century the Vikings appeared, although their usual style was to appear quickly, plunder, and disappear, preferably attacking undefended locations. The Vikings raided places along the coastline of England and France, with the greatest threats being in England. They would raid monasteries for their wealth and lack of formidable defenders. They also utilized rivers and other auxiliary waterways to work their way inland in the eventual invasion of Britain. They wreaked havoc in Northumbria and Mercia and the rest of Anglia before being halted by Wessex. King Alfred the Great of England was able to stay the Viking invasions with a pivotal victory at the Battle of Edington. Alfred defeated Guthrum, establishing the boundaries of Danelaw in an 884 treaty. The effectiveness of Alfred's 'fleet' has been debated; Kenneth Harl has pointed out that as few as eleven ships were sent to combat the Vikings, only two of which were not beaten back or captured.

The Vikings also fought several sea battles among themselves. This was normally done by binding the ships on each side together, thus essentially fighting a land battle on the sea. However the fact that the losing side could not easily escape meant that battles tended to be hard and bloody. The Battle of Svolder is perhaps the most famous of these battles.
As Muslim power in the Mediterranean began to wane, the Italian trading towns of Genoa, Pisa, and Venice stepped in to seize the opportunity, setting up commercial networks and building navies to protect them. At first the navies fought with the Arabs (off Bari in 1004, at Messina in 1005), but then they found themselves contending with Normans moving into Sicily, and finally with each other. The Genoese and Venetians fought four naval wars, in 1253–1284, 1293–1299,
1350–1355, and 1378–1381. The last ended with a decisive Venetian victory, giving it almost a century to enjoy Mediterranean trade domination before other European countries began expanding into the south and west.
In the north of Europe, the near-continuous conflict between England and France was characterised by raids on coastal towns and ports along the coastlines and the securing of sea lanes to protect troop–carrying transports. The Battle of Dover in 1217, between a French fleet of 80 ships under Eustace the Monk and an English fleet of 40 under Hubert de Burgh, is notable as the first recorded battle using sailing ship tactics. The battle of Arnemuiden (23 September 1338), which resulted in a French victory, marked the opening of the Hundred Years War and was the first battle involving artillery. However the battle of Sluys, fought two years later, saw the destruction of the French fleet in a decisive action which allowed the English effective control of the sea lanes and the strategic initiative for much of the war.

Eastern, Southern, and Southeast Asia: The Sui (581–618) and Tang (618–907) dynasties of China were involved in several naval affairs over the triple set of polities ruling medieval Korea (Three Kingdoms of Korea), along with engaging naval bombardments on the peninsula from Asuka period Yamato Kingdom (Japan).
The Tang dynasty aided the Korean kingdom of Silla (see also Unified Silla) and expelled the Korean kingdom of Baekje which were supported by Japanese naval forces from the Korean peninsula (see Battle of Baekgang) and helped Silla overcome its rival Korean kingdoms, Baekje and Goguryeo, by 668. In addition, the Tang had maritime trading, tributary, and diplomatic ties as far as modern Sri Lanka, India, Islamic Iran and Arabia, as well as Somalia in East Africa.
From the Axumite Kingdom in modern-day Ethiopia, the Arab traveller Sa'd ibn Abi-Waqqas sailed from there to Tang China during the reign of Emperor Gaozong. Two decades later, he returned with a copy of the Quran, establishing the first Islamic mosque in China, the Mosque of Remembrance in Guangzhou. A rising rivalry followed between the Arabs and Chinese for control of trade in the Indian Ocean. In his book Cultural Flow Between China and the Outside World, Shen Fuwei notes that maritime Chinese merchants in the 9th century were landing regularly at Sufala in East Africa to cut out Arab middle-men traders.

The Chola dynasty of medieval India was a dominant seapower in the Indian Ocean, an avid maritime trader and diplomatic entity with Song China. Rajaraja Chola I (reigned 985 to 1014) and his son Rajendra Chola I (reigned 1014–42), sent a great naval expedition that occupied parts of Myanmar, Malaya, and Sumatra.
In the Nusantara archipelago, large ocean going ships of more than 50 m in length and 5.2–7.8 meters freeboard are already used at least since the 2nd century AD, contacting India to China.: 347 : 41  Srivijaya empire since the 7th century AD controlled the sea of the western part of the archipelago. The Kedukan Bukit inscription is the oldest record of Indonesian military history, and noted a 7th-century Srivijayan sacred siddhayatra journey led by Dapunta Hyang Sri Jayanasa. He was said to have brought 20,000 troops, including 312 people in boats and 1,312 foot soldiers.: 4  The 10th century Arab text Ajayeb al-Hind (Marvels of India) gives an account of an invasion in Africa by people called Wakwak or Waqwaq,: 110  probably the Malay people of Srivijaya or Javanese people of Mataram kingdom,: 27 : 39  in 945–946 CE. They arrived at the coast of Tanganyika and Mozambique with 1000 boats and attempted to take the citadel of Qanbaloh, though eventually failed. The reason of the attack is because that place had goods suitable for their country and for China, such as ivory, tortoise shells, panther skins, and ambergris, and also because they wanted black slaves from Bantu people (called Zeng or Zenj by Arabs, Jenggi by Javanese) who were strong and make good slaves.: 110  Before the 12th century, Srivijaya is primarily land-based polity rather than maritime power, fleets are available but acted as logistical support to facilitate the projection of land power. Later, the naval strategy degenerated to raiding fleet. Their naval strategy was to coerce merchant ships to dock in their ports, which if ignored, they will send ships to destroy the ship and kill the occupants.
In 1293, the Mongol Yuan dynasty launched an invasion to Java. The Yuan sent 500–1000 ships and 20,000–30,000 soldiers, but was ultimately defeated on land by surprise attack, forcing the army to fall back to the beach. In the coastal waters, Javanese junks had already attacked the Mongol ships. After all of the troops had boarded the ships on the coast, the Yuan army battled the Javanese fleet. After repelling it, they sailed back to Quanzhou. Javanese naval commander Aria Adikara intercepted a further Mongol invasion.: 145 : 107–110  Although with only scarce information, travellers passing the region, such as Ibn Battuta and Odoric of Pordenone noted that Java had been attacked by the Mongols several times, always ending in failure. After those failed invasions, Majapahit empire quickly grew and became the dominant naval power in the 14–15th century. The usage of cannons in the Mongol invasion of Java,: 245  led to deployment of cetbang cannons by Majapahit fleet in 1300s. The main warship of Majapahit navy was the jong.