SAN JUAN TEOTIHUACAN, MEXICO—On 16 January 378 C.E., a stranger arrived in Tikal, a large Maya city in what is now northern Guatemala. His name was Sihyaj K’ahk’ (SEE-yah Kak), or Fire is Born, and he was likely a mighty warrior from a distant land. Many archaeologists think he hailed from Teotihuacan, a metropolis of 100,000 people about 1000 kilometers northwest of Tikal, near today’s Mexico City. And he may have come with an army. The stone Maya monuments that record Sihyaj K’ahk’s arrival don’t say why he came or how he was received by Chak Tok Ich’aak, or Jaguar...