Tonga has flown essentially the same flag since 1875: a red field with a white canton carrying a red cross.
Adopted with the constitution
King George Tupou I introduced the flag on November 4, 1875, the same year the Tongan Constitution was proclaimed, which wrote the flag into the country's legal framework. Later adjustments were minor and left the core design alone.
Tonga was a British protectorate from 1900 until 1970, and the flag came through that period untouched. Britain did not impose a change, and when Tonga regained full independence in 1970 the same flag carried on.
The cross and the field
The red cross in the white canton is a Christian symbol, and Christianity has been central to Tongan culture since missionaries arrived in the 19th century. The red field is read as the blood of Christ and the white canton as purity and hope.
Handling and display
The flag flies on government buildings and at national celebrations such as Tongan National Day. It is hoisted in the morning and lowered in the evening. It should not touch the ground, it is folded properly when stored, and commercial use is not permitted.
Crosses on other flags
A cross on white recalls the emblem of the Red Cross, and crosses appear on national flags across the Christian world. Tonga's particular arrangement, a small canton on a red field, is its own, and it sets the flag apart from its Pacific and Polynesian neighbors, which draw on different traditions.
References
[1] International Federation of Vexillological Associations, "The Symbolism of Flags", Vexillology Journal.