Here's here are some synopses and general points that I can elaborate on if you wish:
In year 170, the first Andromedan ship sighted in our galaxy was encountered by the Kzinti Light Carrier Typhoon in an asteroid field. After that, other Andromedan ships were sighted from time time, sometimes evading contact, sometimes attacking without provocation. Several ships of different races were confirmed destroyed by Andromedan raiders, several other unaccounted for ships are also suspected to be victims of marauding Andros. Over the decade of year 175-185, the Andromedans shifted from a curiousity to a menace as their ships began attacking others on sight.
The Andromedans launched a full-scale invasion of the Milky Way galaxy in year 188. Over the next ten years, large numbers of Andromedan ships arrived in our galaxy and created considerable mayhem and destruction. Since the flight time from Andromeda is on the order of 200 years, the invasion was fully commited before its first recon ships arrived.
The Andromedans apparently built up their strength during the General War and launched their attack only when the galactic forces were exhausted. The Romulans were hit particularly hard by the Andromedans, and the Lyran Democratic Republic was completely annhilated by them. At the height of their power (year 197) the Andromedans had reduced the Romulan, Gorn, ISC, Lyran, and Hydran Empires to small areas around their home systems.
Eventually it was discovered that there were not all that many Andromedan ships. The Andros had created a strategic transportaion network(known as the Rapid Transport Network or RTN) of pre-surveyed routes along which their ships could move at warp 15, allowing them to concentrate their forces at key points. Survey ships discovered the first of these bases in year 195, and by year 198 the network had been heavily disrupted as the scouts and survey cruisers hunted down the bases for the cruisers to destroy.
Finally in year 201, the galactic forces launched a three-pronged attack on the Lesser Magellanic Cloud, destroying the only starbase operated by the Andromedans in year 202. This broke the back of the invasion as it apparently cut off their only direct route from their own galaxy. After that time, the Andros became merely a nuisance as renegades and a few new arrivals caused local disturbances.
Absolutely nothing is known about the Andromedans as beings. No one has ever seen an Andros and lived. Their boarding parties are composed of robotic combat systems, leading some to theorize that the Andromedans themselves are robots or computer software.