Looking for sources: First Messenian War

You and me and the entire field of Classics...

You're very right to worry that there may be few sources. In fact there are practically none, and some scholars doubt whether this war even happened at all. The sum of our evidence is these two fragments from the poetry of Tyrtaios (later 7th century BC):

...to our king, the friend of the Gods, Theopompos, through whom we took spacious Messene, Messene so good to plough and so good to plant, for which there fought ever unceasingly nineteen years, keeping an unfaltering heart, the spearmen fathers of our fathers, and in the twentieth year the enemy left his rich lands and fled from the great uplands of Ithome. (fr. 5)

...galled with great burdens like asses, bringing to their lords under grievous necessity half of all the fruit of the soil. (fr. 7)

Since Tyrtaios wrote his war songs to encourage the Spartans in their next war with the Messenians, we don't know whether he is giving us facts or mere propaganda. While later Greeks and Classical scholars took his "fathers of our fathers" at face value and dated the First Messenian War two generations before the Second, it's also easy to read the phrase as a generic "a long time ago". While Tyrtaios gives the impression of a wholesale subjection of Messene, it's likely that the first conflict was fought over smaller stakes, and that only the second war resulted in the actual conquest of the whole region. In short, practically nothing Tyrtaios tells us is solid ground for a reconstruction of the conflict.

It's only in much later sources that we get details of what supposedly happened in this war. Our main evidence comes from Diodoros' Library of History (specifically the fragmentary book 8) and Pausanias' Tour of Greece 4.4-24. These authors tell us that they based their account on two lost authors dated to the 3rd century BC: the historian Myron of Priene and the poet Rhianos of Bene. But the problem is that we have no idea where these authors got their information. The only thing we can see clearly from the way the story comes down to us is that they did not have an independent Archaic account of the war. Their narrative is fantastical, epic and riddled with anachronisms; it is not likely that we can trust any part of it as historical evidence. It should be read in the context of the Messenians' desire for an ethnic origin story after their liberation in 370 BC. Even Pausanias himself already casts doubt on the value of Myron's account; should we overrule him without seeing the source for ourselves?

The result is that we're left with practically nothing to go on. The First Messenian War is not archaeologically visible, unless we assume that all evidence of violence must relate to it, and ignore all other possibilities; Anthony Snodgrass rightly criticised archaeologists who excavated a house in Messene with fire damage datable to c. 700 BC and presented it as evidence of the war. At best, we can say that the First Messenian War is a conflict that allegedly happened, may have taken as many as 20 years, and probably resulted in the subjection of some or all of the region to Sparta, including the imposition of heavy taxes. The rest is mostly a deliberate effort on the part of Spartans and Messenians to mythologise the past.

The main modern works to read on this are Nino Luraghi's The Ancient Messenians (2008) and Daniel Ogden's Aristomenes of Messene: Legends of Sparta's Nemesis (2004).