We hear a lot about the Aztecs, but the Purepecha Empire next door was almost their equal in size and kept the Aztecs at bay militarily. Was the Purepecha Empire a state of similar standing? Why don't we hear more about it?

The P'urépecha state, Irechecua Tzintzuntzani (literally "The Kingdom of Tzintzuntzan") was the third largest state in the Americas at the time of Spanish conquest, behind the Inca and Aztec empires. They occupied a territory of 75,000 square kilometers (about the size of Scotland) at the time of Spanish arrival, and were larger than that in their past. They were perhaps the most centralized state in Mesoamerica, had a sophisticated network of border fortifications, and are one of the few pre-Columbian states to not just successfully resist an Aztec Invasion, but soundly defeat them. So I would say, yes, they were in a "state of similar standing" to the Aztecs.

The reason we don't hear more about it is that it hasn't been studied as much. In contrast to the Aztecs, our historical sources on the P'urépecha are much thinner. The main historical source, the Relacion de Michoacán, is pretty much all there is on the pre-Columbian history. There are few minor sources, like the Lienzo de Jicalán or La Memoria de Don Melchor Caltzin, but these discuss specific, isolated events. They don't provide the overview narrative history that we need to flesh in lots of the details. While there are a few colonial era sources, like the Relaciones Geograficas, there isn't really anything comparable to the numerous Spanish chroniclers and native historians that we have for the Aztecs. This is largely due to the fact that the Aztecs provided a stiff resistance to the Spanish, and their state became the backbone of the Spanish colonial administration after the conquest. The P'urépecha, by contrast, surrendered without fighting because they were dealing with the smallpox outbreak and a coup when the Spanish entered the territory. Rather than forming the basis for a colonial administration, their political institutions were largely cut out of the colonial system in the 17th century when the Spanish decided to make their own regional capital (modern day Morelia) to bypass native authority.

On the archaeology side, the region has been heavily understudied as well. Early work started with Alfonso Caso and some of his disciples doing some cursory excavations at the civic-ceremonial core of the capital, Tzintzuntzan, in the early 20th century. The region was largely ignored from that point until the late 1970s when Hellen Pollard did her PhD dissertation on the urban layout of the capital. Pollard did some further work on some of the smaller cities on the west side of the lake basin in the 1980s, and at the same time her colleague Shirley Gorenstein did some archaeological work on the northeastern frontier. There were a few other historians and archaeologists during this period, but prior to the start of the 21st century I can literally count on my hands the number of people who seriously studied the region in an academic capacity. One can contrast this to the Maya region where you can't throw a rock without hitting another archaeologist.

As to why the region is understudied, there's a kind of feedback loop in the research of Native American cultures. People look at the (lack of) existing research, conclude that the region is not interesting or doesn't have anything worth pursuing, and therefore they don't study it. It's much easier to get grant money studying a culture like the Aztecs, or the Maya, where you know you'll find something. Since nobody researches it, nobody finds anything, and so nobody looks. Up until very recently, the general conclusion was that despite the size of the empire the area was a cultural backwater that only recently acquired urban civilization. In fact, if you pick up any book or article written before about 2010 it would tell you that there were no cities in Michoacán prior to the formation of the P'urépecha state, and that the P'urépecha lacked even a pictographic script like the Aztecs. (We now know neither of these assumptions were true.)

Luckily, this situation is now changing as there is a renewed interest in the region. There's been a lot of recent research on the culture, but unfortunately little of this research has been published in English. A French team organized through Centre d'Études Mexicaines et Centroaméricaines (CEMCA) conducted a decades long project studying the Zacapu Basin (the lake basin immediately north of Lake Pátzcuaro where the empire formed). This project has published extensively in both French and Spanish, but almost none of their work is available in English. An American team organized principally through Colorado State University has for several years been studying an extremely large city on the east side of the Lake Basin that was not previously known to archaeologists. The results of this research are likely to completely rewrite our history of P'urépecha state formation, but unfortunately little of that research has been published as of yet, outside of PhD dissertations and a few cursory overview articles. There's also a Mexican team organized through UNAM who is actively working on a site south of the Lake Pátzcuaro basin which is just now getting started.

The issue is that so much of this research is so new that it will take time for the academic arguments to shake out into a new consensus. I imagine that, in a few years (or decades), we will probably have enough information for somebody to write a new synthesis.

I will add sources and other info to this post later if people are interested. There are a few English language articles available that can provide a sense of the current scholarship. But right now it's really late where I am so I'm going to bed and I'll add more info tomorrow.

EDIT:

Okay, I'm awake and I've had my coffee. First, I want to direct people to the relevant section of the AskHistorians book list. The two books on there are the standard introductory works, but both are out of date by this point. I'd still recommend them for people coming fresh to the topic with no background knowledge, as there isn't really anything comparable that incorporates new research.

Another recent book that is a fantastic read is Afandor-Pujol's book analyzing the main historical source. This book is about the early colonial period more than the pre-Columbian state, but it provides the best breakdown of how to view the primary source material I've seen. It is this book which argues (rather conclusively) that the P'urépecha did indeed have pictographic writing like the Aztecs.

There have been two recent PhD dissertations that provide the most comprehensive overview so far of the American lead research at the (probably misnamed) site of Angamuco on the east side of the Lake Pátzcuaro Basin. The first, by Anna Cohen, looks at the site's ceramic chronology. The second, by Rodrigo Solinis-Casparius, looks at the site's road network. You can download both as pdfs through the link provided (the pdf link itself is on the left hand side of the page). Being dissertations, these book-length documents are very data heavy. If you decide to read them, you may want to skip over most of the body chapters and focus on the intro and conclusion chapters, unless you have a burning curiosity for the nuts and bolts of archaeological research.

Other articles on this research are unfortunately locked behind paywaylls. If you have access to an academic library, there's a chapter in a book written by the project's director which provides an overview of the site. There has also been a recent publication on architecture at the site, also behind a paywall here.

A few of you also asked for Spanish language sources. The French project working in the Zacapu basin has published a great overview in Spanish available here. Another overview of the project offering a different perspective was written by Charlotte Arnauld (2015 - "Evolución de la ocupación humana en el centro-norte de Michoacán" in Génesis, culturas, y espacios). I can't find a copy of it online, unfortunately. The entire full text of the Relacion de Michoacán is also available on the Colegio de Michoacán's website along with other helpful information on things like geography.

The most active Mexican archaeologist studying the P'urépecha is Jose Luis Punzo Diaz. Most of his research up until now has been looking at the Epiclassic Period (600-900 AD) site of Tingambato, which predates the formation of the state by many centuries. His articles on this, which you can find by searching his name on Google Scholar are largely on really specialized topics. Results of dating techniques, geophysical prospection, ceramic analysis, etc. There isn't much in the way of synthesis articles. He has recently started a new project south of the Lake Pátzcuaro basin looking at a site closer to the period of the P'urépecha state, but to my knowledge there haven't been any big publications on that yet. He just started a few years ago.

If anyone has any specific questions about any of this research, or anything else on the P'urépecha, I will be happy to answer them.