Let's see
Did the marines save Sabo, Ace or Luffy in their childhood island? No, Dragon and Shanks did
Did the marines do something to Helmeppo, who was attacking random people on the streets for being the son of a marine? No, Zoro did, and got locked up for it, only for Luffy and Zoro then save the entire city from Morgan
Did the marines save Usopp's Island? No, Straw Hats did. In fact, the villain literally used the marines to fake his death and do whatever he wanted.
Did the marines save the Baratie being attacked by Krieg's crew? No, Mihawk (of all the people lmao, and he was bored) and the Straw Hats.
Did the marines save Nami's island from Arlong? Nope, they let them suffer for over a decade, had a deal with Arlong and even came to steal Nami's money, the one trying to save the island. Who saved them in the end? Straw Hats, no way bro.
Did the marines save Arabasta? nope.
Did the marines save Dressrosa? nope.
Did the marines save Wano? nope.
Did the marines comit genocide and killed the entire island of Ohara? Yes. Why? Celestial Dragons told them too.
Did the marines help save slaves in Sabaody? nope, Straw Hats did.
I can keep going.