The Raid on Liberio was a preemptive strike carried out by the Scout Regiment against the Marleyan military during a festival hosted by the Tybur family in Liberio. The attack was commenced by Eren Jaeger shortly after Willy Tybur formally declared war on Paradis Island.
Prelude: In 851, Zeke Jaeger sent Yelena and Onyankopon of the Anti-Marleyan Volunteers on an infiltration mission. They integrated themselves as part of the crew of a Marleyan ship that went to Paradis. When the ship was captured by the Scout Regiment, Yelena killed the captain and agreed to Commander Hange Zoë's suggestion to talk.
The Anti-Marleyan Volunteers: In 851, Zeke Jaeger sent Yelena and Onyankopon of the Anti-Marleyan Volunteers on an infiltration mission. They integrated themselves as part of the crew of a Marleyan ship that went to Paradis. When the ship was captured by the Scout Regiment, Yelena killed the captain and agreed to Commander Hange Zoë's suggestion to talk.
Nations in decline: Three years later in 854, the Armored Titan suffered an unceremonious near-defeat at the hands of the Mid-East Allied Forces' anti-Titan artillery, serving as a grim reminder to the Marley military that the era of the Titans was coming to an end. Following Marley's victory over the Allies, the leaders of the Marleyan military gathered to discuss the apparent threat to their global influence. Commander Theo Magath, noting the Allies' superior navy in the recent war, was of the opinion that reliance on the power of the Titans could not guarantee victory in ground-based warfare any longer, especially given the rising advent of military aircraft which would render the Titans completely obsolete in battle.
A new Helos: Soon afterward, the Tybur family arrived in Liberio where Commander Magath met with Willy Tybur, the head of the family. Together the two discussed the declining state of their nation, comparable to the empty legend of Helos the slayer of the Devil, and the suffering of the Eldian people, both crises which the Tybur family were in some way responsible for. As Magath had suspected, the Tybur family had been acting as the true ruling head of Marley, though in matters of both Marley's military policies and the subjugation of the Eldians the Tyburs remained uninvolved observers.
Source: Raid on Liberio (Anime), available under CC BY-SA 3.0.