The Portuguese made their appearance in Southeast Asia when they conquered Malacca on 24 August 1511. Their Iberian neighbours the Spanish started establishing themselves in the Philippines with the arrival of Miguel López de Legazpi's expedition on 13 February 1565, from Mexico, more than half a century later. (Ferdinand Magellan's expedition in 1521 was the first time Europeans landed on a Filipino island.)
Back in Europe, the death of the young king Sebastião in 1578 at the Battle of Three Kings in Morocco triggered the War of the Portuguese Succession. This led to the Union of the Portuguese and Spanish Crowns between 1580 and 1640 under a series of kings named Philip. In Asia, this meant that starting in 1580 there was significantly more coordination between the Portuguese strongholds of Goa and Malacca and the Spanish-controlled Philippines.
Diogo Veloso, born 1558 in Amarante east of Porto, was a Portuguese adventurer who reached Cambodia via Malacca and befriended in Longvek (the Cambodian capital from 1528 to 1594) both the Cambodian King Satha and the Spanish explorer Blas Ruiz de Hernán González. There are varying accounts of the history of this period, but King Satha is said to have sent Veloso to Manila in 1593 asking for Spanish help to defend Cambodia, which had been in a long period of decline since the heyday of Angkor Wat. The Siamese King Naruesan of Ayutthaya conquered Longvek that year before the Spanish decided to do anything, and the Cambodian court fled to Lan Xang, another kingdom further inland. The Spanish appear eventually to have sent three ships with Spanish and Mexican soldiers along with Japanese and Filipino auxiliaries, but it is unclear what they achieved and whether they were simply made use of by different political factions on the Cambodian side struggling over succession issues.
Diogo Veloso and Blas Ruiz, by venturing to Lan Xang in search of their friend King Satha, thus became the first Europeans to set foot in what is today Laos.