The following story is based on Hellsing: Ultimate and contains mayor spoilers to this story.
In 1943, the British Secret Service discovered the existence of German Initiative, Special Order #666. The order, given by Adolf Hitler to Montana Max, was to create an army of vampires. Together with werewolf Captain Hans Günsche and scientist Avondale Naypeer, Montana led a team of German scientists and occulists that would used the remains of Mina Harper (which were exhumed in 1941 after the Nazis conquered Hungary), to recreate vampires. The intel told the British Secret Service that Avondale's attempts to create vampires have failed so far as his test subjects (prisoners of war) turn into ghouls, shortly after the artificial vampirization process. Arthur Hellsing however realized the danger of an army of ghouls which would spread like plague if unleashed, send a young transhuman, Walter Dornez and his enslaved vampire Alucard to Warshaw, Poland in 1944 after discovering the vampirization facility.
During their assault, Walter and Alucard would destroy the facility and apparently ended Special Order #666. Unbeknownst to all, Montana, with the help of the captain, wooed Walter to become a secret agent for Millenium and let them go. With Walter's help, most of the members of Millenium traveled to Brazil. Montana, having the title of Major was however summoned to Berlin, where he almost died at the hands of Russian soldiers. He was saved however by Avondale who would save his life by cybernizing him, turning him into a cyborg, immortalizing him. Now, with all the time in the world, The Major was able to plan the death of the vampire king Alucard, proving the superiority of humans over vampires.
In the following 50 years, Avondale was able to vampirize most of The Major's men, creating a vampire army known as the Letzte Bataillion. With the help of Walter, Alucard was re-awakened by the new head of the Hellsing family, Integra Hellsing. He also started with the vampirization of transhumans, namely speedster Luke Valentine, card manipulator Tubalcain Alhambra and telepath Zorin Blitz. 10 years after the awakening of Alucard, The Mayor started to test Alucard's abilities, by increasing vampire attacks in England and provoke a battle between father Anderson and the vampire. Other tests involved a raid on the Hellsing Mansion and a siege on Alucard and his servant Seras Victoria in Rio De Janeiro, by Alhambra. Once he learned enough, the Mayor lured Alucard to open sea, with Rip Van Winkle. Although she did die, she was successful in her mission, trapping Alucard on a boat in the middle of the ocean.
Operation See Löwe II
With Alucard out at sea, The Mayor attacked London with his Zeppelins. He destroyed large parts of the London using V3 rockets, before unleashing the Letzte Bataillion on London, killing millions. After they apparently kidnapped Walter, Millenium vampirized the transhuman and making him their trump card. An unknown Millenium agent in the US, a presidential aid, caused a massacre during a cabinet meeting, killing fourteen people, the president included. With London burning, the Vatican sent the Iscariot Organization and Papal Knights to fight these nazi vampires, although their commander, Archbishop Enrico Maxwell turned it into a crusade. With London being reshaped to a hellish war zone, Alucard was able to move the ship he was trapped on and returned to London. While most of The Major's army ravaged London, he sent to Zorin Blitz to the Hellsing Mansion but ordering her to wait for further orders. She ignored these orders however and attacked the mansion. While she was able to destroy the mansion, she and her battallions were killed by Seras, who became a full-fledged vampire in the process.
When Alucard had returned to London, he was ordered to revert to Level 0 form, leaving him both at his strongest as at his most vulnerable. Using his familiars, he made short work of the vampire army as well as the Papal Knights but in this battle, he lost huge parts of his familiars. He was eventually challenged again Father Anderson, who eventually turns himself into a Monster of God to win the battle. Although he was unable to kill Alucard, he was able to eliminate all his familiar before dying. Soon after, the vampirized Walter Dornez took over, almost beating Alucard, unintentionally forcing Alucard to absorb the blood of London's deceased to refill his health. The Mayor's plan finally came to succession by this act, as he used the transhuman Schrödinger who sacrificed, to poison the blood. Schrödinger can only exist if he is aware of himself, amongst 3 million other souls, he can't and so, Alucard can't. Although all his allies were dead or soon to die (Seras killed the Captain, a reformed Walter was about to kill the Doctor), he was happy to finally have beaten Alucard, dying with no regrets when Integra Hellsing shot him in the head.
The Zeppelin Affair was the worst terrorist attack, till the Omega Event and caused reform in the Vatican. It also exposed the existence of vampires to the world.