Here is my theory, I think people just put bounties on Mihawk and the marines took it up.
by this I mean, there’s a specific type of role that the marines SHOULD have but we’ve never seen them actually have. That’s extradition.
If for example Higuma the Bear from Luffy’s village were to manage to escape into the sea after killing Luffy, even without a pirate flag, he becomes the marines responsibility. If Goa kingdom only has an army and no navy, then it becomes the Marine’s responsibility to hunt down Higuma since Goa is part of the World government so the services of the Marines should extend to them even if Higuma isn’t actually a pirate. Since Higuma is on the sea, the marines would have to be the ones to Capture him and extradite him back to Goa kingdom to be punished in Goa.
I think this is what happened with Mihawk.
mihawk Defeated whoever was the strongest swordsman on his home island and according to who that was and how the fight went, Mihawk could’ve easily become wanted by his own Country’s government. And even if he didn’t get a bounty from this like maybe it was a fair duel that was recognized by the public, the fact that mihawk decided to go to sea to beat other countries’ strongest swordsmen increases the chance that he eventually would get a bounty like this.
imagine this, Mihawk arrives at some island and finds their strongest swordsman and tells them he wants a fight. He doesn’t say “a fight to the death”, he just says a a fight. The strongest swordsman knowing he’s the strongest is arrogant and just accepts. He gets beaten and then
1) is killed and immediately Mihawk gets a murder charge
2) gets severely injured and goes to complain to his government that he was attacked abruptly
3) gets defeated and forced to submit with little injury but then has a vendetta against mihawk and tries a sneak attack and gets killed or he goes and complains again to his government and makes some shit up to make Them give Mihawk a bounty
what I’m saying is there’s not many Zoro type people in the world who would just take getting defeated so honorably especially if they were already the strongest on their island and were proud of their strength
And if Mihawk kept doing this for like dozens of islands ALL OVER the world and a number of those Islands were part of the World Government, every time Mihawk escaped to sea, he would become the Marines’ problem and even if he hasn’t done pirate stuff, he still has extradition on his head from multiple countries and the marines job is extradition
basically Mihawk’s bounty is entirely built on grudges since he’s going around crippling multiple countries’ armies since he keeps defeating their best men.
And After a while the marines realized they couldn’t catch the guy and just gave up and made him a warlord. Or he just got tired of beating chased and offered the marines his services as long as they stopped chasing him and he could keep going around beating everybody.
If you notice, in this scenario, Mihawk never has to even beat one marine. Not even destroying a single Marine ship. He just has to has to not get caught while still racking up multiple bounties all over the world. And since we know random people like Issho and Greedbull can exist on random islands all over the world, Mihawk isn’t just fighting fodder, he could legit be beating people who are the lynch pin to a country’s military power. Like he finds a country where their big military hero is an Issho type guy and beating just that guy and possibly injuring him beyond being able to fight again, completely throws off the country’s military and then once again bounty for extradition is put on him.
Therefore as an example, this is how Mihawk’s bounty would be formed. (I’m gonna say it’s 2 billion). Consider that a low level bandit in East blue could get 8 million bounty.
The Navy trying and failing to catch him-30 million
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20 WG allied countries put bounties of minimum 80 million on his head. I’m using 20 countries because there’s 170 countries in the WG and Mihawk traveling the world could have easily managed to find 20 with reasonable people to beat. I’m also using 50 million because we saw that Dressrosa, a “poor country” can manage to make 10 billion and Nico robin said 500 million is enough to buy a small country. So it’s clear that Dressrosa being “poor” is in regards to being a World government Ally. A poor WG country is different from just a plain poor country. Therefore 80 million for a bounty is reasonable for a WG country. So we have 1.6 billion.
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30 Non Allied countries put bounties of like 10 million on his head. I’m using 10 million again because Poor countries but it’s possible for some countries not to be poor and not in the WG. The marines aren’t actually obligated to these countries so they don’t have to actually consider these bounties BUT since they can’t ignore that this happened, they’ll just take half. So only 150 million comes from this sine it’s just portrayal and the marines don’t have to consider it.
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General hype around his strength since the Marines know he’s the Strongest swordsman will add in the remaining 220 million.
There we have 2 billion made from simply fighting people and evading the marines without any explicit pirate behavior of any kind.
And I’m saying it’s 2 billion AFTER the warlords have been abolished since in this case, Countries that previously could not give Mihawk a bounty since he’s a government agent now get the right to give him a bounty. By that I mean among those 20 WG countries, a number of them had Mihawk walk in and destroy their best men but since Mihawk was Warlord, they couldn’t do shit until now.
@Mochiman