Which of the New Generation Pirates crews are strong enough to survive and/or defeat Marineford?
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Crews Strengths
Alexander's Crew: Numbering 7,500 pirates and 75 powerful Devil Fruit users. Former allies of Whitebeard fighting among him.
Tatsu's Crew: A very powerful armored fleet. Battle harden samurai warriors from Wano and a Devil Fruit similar to Fujitora.
Lucifer's Crew: Very ruthless demons and numbering more then Marine forces. Seven very dangerous commanders and very dangerous Devil Fruit abilities.
Akbar's Crew: Massive Zoan users army and Akbar's Leviathan Devil Fruit.
Was the Marineford War an epic fail for the Marines?
Whitebeard dying only to give away his power to Blackbeard, giving him the potential to rise from Shichibukai to Yonkou level. Way more evil and dangerous than WB will ever be.
All the islands that WB had in his control, which were very peaceful, were taken over by other pirates. This means more work for the marines since they have to expend more resources and time to provide protection for those islands.
The death of Ace: They eliminated Roger's bloodline and that was great and all. But honestly all he ever did in the series so far was search for BB. With his 'I won't run away' motto, he would've gotten in a much bigger mess at some point even if the marines or BB didn't do anything.
Ace's DF was claimed by a revolutionary, yet again another fail for the World Government
WB's 'One Piece exists!' speech incited the next generation of pirates, which was totally the opposite of what Sengoku wanted to do. Surveillance Den Den Mushi - you da real MVP
If the war never happened, Luffy would've never gone to Impel Down and Marineford, and made such an impact on the world. He's made allies of the WB pirate alliance, several Shichibukais, some Impel down escapees and the Revolutionary army.
Even though Luffy lost few years of his life during the whole process, with the addition of the allies, he gained immunity to poison and kick-started his Conqueror's Haki growth.
Most importantly, this was the best chance the Marines had in taking down the future king of the pirates. Never again will they be able to fight the already-fatigued-from-Impel-Down-No Haki-and-reckless version of him.
Law mentioned that the Marineford War was only a prelude to the future battles that is yet to come. He's right: Regardless of what Pacifista or human experiments or ancient weapon that the WG does to expand their army, it won't be enough to stop the humongous alliance (it will be way bigger than WB's fleet) that Luffy will bring in the final war.