The history of Brittany is quite complex, here are only some of the few important dates.

7000 - 500 BC: The first people living in Armorica (ar mor = "the sea" in Breton language) belonged to the megalithic civilization.
They built those incredible stone monuments that you can find all over Brittany today which include:
- Menhirs, which are often isolated stones (such as at le Champ-Dolent, near Dol-de-Bretagne) or grouped into lines (Carnac)
- Cromlechs (stone circles)
- Dolmens (at Locmariaquer), covered alleys or even Cairns (such as in Bamanez in Plouezoc’h, near Morlaix)
- Tumulus (Gavrinis in the Gulf of Morbihan, Bourbriac). 
Their real origin and significance are not really known but they play an important part in our legends and beliefs.

500 BC - 200 AD: Celts started to populate Armorica/Breizh at the same time as the rest of Europe.
One of the most famous tribes was the Venetes. Fine sailors, they would be remembered to have fought Caesar and loose the battle only to due to the wind dying out during the battle (50 BC).
They had a strong relationship with the other Celtic tribes of the British Isles and were sharing very similar customs.
Caesar vanquished most of them but romanisation didn't happen to the same extent as in other parts of Europe

300 - 600: But the Bretons you're asking me?
In fact the Celtic tribes had always strong links, sharing similar customs and language. Not having a common enemy they would spend their time battling or commercing between themselves... Relations between Armorica and Brittania (Wales, Cornwall, Scotland and England) were important.
The Bretons living in the British Isles having been invaded by the Romans faced a new invasion coming from the East (Germanic this time).
The Saxons (an English person is still called a Soaz in Breton) pushed the Celtic tribes living on the Isle Westward. So much so that some of them (ie: the Bretons) emigrated over to their Celtic counterparts in Armorica. It seems that their integration into Armorica was easy and peaceful, as their traditions and culture were similar to those of their hosts. Armorica became Brittany and was divided in small kingdoms. They also brought with them their newly acquired religion Christianity, and Breton saints started to convert some of the still druidic tribes.

600 - 900: But the peace and quiet found in Armorica didn't last long for our Bretons.
A new Germanic invasion (the Francs this time) was also pushing from the East. Despite this, slowly but surely the Bretons extended their grip to their new territory to the point they were able to establish a unified Kingdom ruled by our first King: Nominoe.
Charlemagne and his fellow Franc kings/emperors came a few times decimating Brittany (killing women and children as his troops went along) but it is recorded that he maintained that there were such losses in his troops trying to conquer Brittany that he could not sustain campaigns against us. We were at peace for a while.

900-1100 Once again peace didn't last long.
Viking invasions ravaged Brittany. Some of the main cities were burnt to the ground (Vannes/Gwened, Nantes/Naoned). The country was left in a bloody state fighting both the Vikings from the Coast and the Francs inland. Even the English trying to invade us during this period!

1100-1300: From there on, we were a Dukedom, one day linked to the English rule, one day linked to the French/Franc rule.
The dukes tried to play between those 2 rivals and most of the time were able to rule Brittany as a totally independent country. Some of them still refering themselves as Rex (= King) probably in accordance with some irresistible celtic provocation! It's important to note that at this period, with the exception of the high aristocracy, everybody still spoke Breton and we even received our own flag (Croaz Du="Black Cross") from the pope.
The belief that one day we could get rid of both of the invaders was so strong that some of the dukes gave to their children the legendary name of Arthur.

1300-1600: From here, I'm afraid it's downward all the way for little Brittany.
After our 'Succession Wars', where both England and France tried to annex the Dukedom, the old kingdom was left in jeopardy.
Our last duke got the idea to marry his daughter (Anne of Brittany) to the Austrian successor in order not to succumb to any of the 2 parties. The French king didn't really like that very much and asked the pope (probably in exchange for a few favours) to cancel the marriage so that she could marry him.
When the duke died, as he didn't have any other children, the Independent Dukedom was offered (let's say taken:-)) to the French king but stipulating that if any male would be born from this union he would regain the Title of Duke of Brittany. Talk about bad luck... they had a .. girl, who only lived in Paris and upon her marriage gave it all away... However we still kept our parliament and only lost it after the French revolution...

1600-1800: Not much is to be said about this time, there was a few revolts (Red Bonnets, Paper "Timbré", PontKallec) but they were quickly dealt with. It is said that the French revolution started in Brittany, when some "Doléances/Grievances" were sent to the French King. But nowhere else was it so fiercely opposed than Brittany.
To save a king you're telling me?
More to defend the religion, as those revolting wanted to banish Religious Orders and even created a new God ("etre supreme") to replace the Christian religion. The priests that didn't want to pledge for the republic were executed. It didn't go down very well with the population but the counter revolution was crushed in the same bloody fashion as the revolution established itself.

1800-1900: Industrial revolution didn't really affect Brittany and except Nantes/Naoned, most of the population remained farmers, speaking Breton as their everyday language.

1900-2005: Some very inspired educational ministers decided that the Breton language (older than most languages in Europe) was a barbaric language and that for the unity of the Republic, active measures should be taken to get rid of this annoyance. A law was even voted in Paris to ensure legality of the measure. Before those measures it's estimated that in 1900 80% of the west of Brittany was speaking Breton (1.3 million), most of them only speaking this language. 
The First World War sent a lot of Bretons to the trenches, 1/4 of them died. For 4 years, while at war, they learned the French language and brought it back with them.
From this period we kept the term "Plouc", this is a deformation of the breton word "Plou" (church, parish) similar the Irish "Cill". The Bretons not able to speak French used to congregate together by Plou. The French officers started to call them Ploucs (meaning dirty farmers). Long gone were the times we were a proud Celtic nation.
Fortunately for us the 20's and 30's saw the formation of Celtic Circles and the start of Bagads (Breton music bands). After the Second World War, some of the bretons having looked favourably on the occupation of Brittany by the Germans, lead to a total repression against the Breton culture. It was after this period that the greatest decline in Breton speakers was seen.
After 2 decades of forced shame on those who continued to live the Breton culture, the seventies arrived with a new renewal wave, especially for music, and New Age Bards such as Alan Stivell appeared.
Some parents concerned by the rapid decline of the language, enforced by the ruling government, started to create their own private Breton speaking schools, called Diwan (the Seed) financed only by private funds. But with the number of people, having Breton as their first language, dying every year estimated to be around 15000, the prospects for our language are not great.
In the last few years, some public Breton speaking schools supported by the governement are now opening.
We still live in hope.... as the song goes "if for every generation there are still children that recognize themselves to be Breton".