The year is 378 AD and Sihyaj K’ahk’ has just seized control of the Petén Basin, the heartland of what we associate with classical Mayan civilization. But Sihyaj K’ahk’ was not a Mayan, he was in fact from Teotihuacan, which is located in the central Mexican highlands where modern day Mexico City stands today. The event which we currently refer to as the Entrada of 378 AD radically altered Mayan culture to the point that much of what we think of as "Mayan" started in 378 AD. Even 40 years later the significance of what happened in 378 AD was still being memorialized in stone and offerings were still being offered more than 250 years later at a shrine commemorating the event. The fact that we know so little about the Entrada is due to Mayan politics hundreds of years later that conveniently forgot about the huge event that made it all possible.
So what does all of this have to do with Mormon and the battle at Cumorah?
In the second half of the 300's there were several large wars between the Nephites and the Lamanites. Mormon outlines the major events of a war that lasted from 360-367 (cf. Mormon 3-4:15). The next and final war lasted from 375-385 (cf. Mormon 4:16-6:5). There some events recorded for which we do not know the year, but in either 378 or 379 the war takes a turn for the worse and many major cities are lost. As we read in the Book of Mormon, in the beginning of the year 385 the Nephites gathered in the land of Cumorah in a final defense and were defeated and hundreds of thousands killed. It would have been an immense moment in history that greatly changed the course of kingdoms and empires around them.
Apparently this major battle happened just seven years after the Entrada. This is significant because the Petén Basin is one of the leading candidates for the location of the Book of Mormon lands. If this was the location of the Book of Mormon lands then what we know as the Entrada happened in the middle of the final war between the Nephites and the Lamanites. But, as I have mentioned before, calendars are funny things, and a year isn't always a year.
If we assume that Mormon was using the standard Mayan long count calendar then a year, as recorded by him, would have been 360 days, rather than the 365(.24) days in a solar year. This means that a date of 385 recorded in the Book of Mormon may not have been 385 AD according to our calendar. If we account for this then 385 years in the Mayan long count would be ~378.5 +/- 1 years in solar years. Thus, assuming there was no mistake in the reckoning of the date, the final battle at Cumorah may have taken place in 378 AD, the exact same year of the Entrada that marks the transition from pre-classic to classical Mayan civilization. If this is the case then the Entrada was a transformative event in more ways than we now realize.
There is much we do not know about the Entrada. Scholars have only been aware of it for about 20 years, but as one scholar put it "the texts say almost nothing about the circumstances surrounding [the Entrada]." We still have much to learn about that period of history and there may be more than a few surprises left to find.