What was the origin of the city of Venice?

In the Adriatic Sea were a great number of marshy islands, separated only by narrow channels, but well screened and almost inaccessible, inhabited by a few fishermen. To these islands the people of Veneti (a part of Italy, situated along the coasts of the gulf,) retired when Alaric, King of the Goths, ravaged Italy. These new Islanders, little imagining that this was to be their fixed residence, did not, at first, think of forming themselves into one community, but each of the 72 islands continued a long while under its respective masters, and formed a distinct commonwealth.

Adriatic Sea, a name given to the Gulf of Venice.

Commonwealth, a republic, a government in which the supreme power is lodged in the people.

What circumstance caused them to unite?

Their commerce becoming considerable enough to awaken the jealousy of their neighbors, they united in a body for their mutual protection: this union, first begun in the 6th century and completed in the 8th, laid the foundation of the future grandeur of the state of Venice. From the time of this union, fleets of their merchantmen sailed to all the ports of the Mediterranean; and afterwards to those of Egypt, particularly to Cairo, a new city, built by the Saracen princes, on the banks of the Nile, where they traded for spices, &c. The Venetians continued to increase their trade by sea and their conquests on land till 1508, when a number of jealous princes conspired against them to their ruin; which was the more easily effected in consequence of their East Indian commerce, of which the Portuguese and French had each obtained a share.

Conspired, united together in a plot.

What is the signification of Mediterranean?

Inclosed within land, or remote from the ocean. It is more particularly used to signify the sea which flows between Europe and Africa.