The reason behind the war 15 years ago was because Vers being an impoverished, shabby country looked with scorn and jealousy upon Earth, a world with a vast history. The Royal family used the people's jealousy of Earth to maintain order on Vers. The Royal family whipped the Orbital Knights into a frenzy and sent them against Earth. This alone caused the conflict spurring Heavens Fall. Count Saazbaum was sent to Earth as a scout and later his betrothed Viscountess Orlane lost her life. He blamed the Vers Royal family for this and seeks vengeance by making them pay with their own flesh and blood. (AZ: "Before the War")
In July 2014, after waiting 15 years since the end of the First Earth-Mars War that started with Heavens Fall, the presumed death of Princess Asseylum Vers Allusia during her goodwill visit to Earth gave the Orbital Knights a pretext to again, wage war with Earth. (AZ: "Princess of VERS")
In Shinawara, where Princess Asseylum was apparently assassinated, Sir Trillram tried to neutralize the assassins with his Nilokeras to prevent the knowledge that the Orbital Knights ordered the assassination from getting out. The daughter of the leader, Rayet Areash managed to escape and so Trillram pursued her as she sought refuge with the Shinawara High School students and the United Forces of Earth.
As the Wadatsumi made its way to UFE headquarters, it stopped at Tanegashima, a former battlefield of the First Earth-Mars War and the Wadatsumi came under attack by Countess Femieanne and her Hellas. The crew discovered the Deucalion, an abandoned ship.
By December, the Deucalion had reached UFE HQ in Novosibirsk, Siberia and the civilians were brought into the underground shelter. As Count Saazbaum now knew that the Princess was alive, he decided to launch a full out assault on the military base. (AZ: "Childhood's End")
- The picture shown above is from Episode 9 (Darkness Visible) which is before the crew of the Deucalion arrived in Novosibirsk. However, the landing of Saazbaum's castle is already seen, before it did so in the anime.