In 1571, the Ayutthaya Kingdom and Lan Na were Burmese vassals. Having twice defended Lan Xang from Burmese invasions, King Setthathirath moved south to conduct a campaign against the Khmer Empire. Defeating the Khmer would have greatly strengthened Lan Xang, giving it vital sea access, trade opportunities, and most importantly, European firearms which had been growing use since the early 1500s. The "Khmer Chronicles" record that armies from Lan Xang invaded in 1571 and 1572, during the second invasion King Barom Reacha I was slain in an elephant duel. The Khmer must have rallied and Lan Xang retreated, Setthathirath went missing near Attapeu. The Burmese and Lao Chronicles record only the presumption that he died in battle.