Crocodile doesn't even know the basics of Haki and was defeated by Luffy. How did such a weakling became a warlord?
The exact nature of how crocodile was selected to be a Warlord is not known.
However this is what Oda has to say about it
Similar to Luffy, back when he was young, Crocodile's name spread across the seas with incredible momentum, but soon after he was admitted into the Shichibukai in his mid-20's, he tried to fight Whitebeard and was completely and utterly crushed by him.
Source: SBS Vol.78
This seems like a rant question to me. So addressing those issues.
Crocodile was one of the strongest Warlords. He had a powerful Logia fruit with great control over its abilities. He could use its effects over large areas of land. His strength is compared to Jinbei and Ace in the Impel Down Arc.
Bartholew Kuma, Gecko Moriah, Jinbei may or may not have Haki.
Crocodile was one of the most important villains in the One Piece Universe. He beat the protagonist not once but twice. Had an interesting power, introduced us to the Shichibukai and created a very vast organization almost taking over a country single handed.
Also, I am not sure how you know Crocodile doesn't have Haki? He considered himself strong enough to attack Whitebeard head on. We simply don't see him have any need of it to use anytime he has been onscreen.
Moving out of the world, Oda had thought of Haki before the series started, but hadn't used it before the Sky Island Arc (Mantra as observation haki). We have at least one clear example of this. Shanks scaring of the sea king. Mihawk may have used Observation Haki to beat the shit out of Zoro on Baratie too.
These may be the reasons that he had left them for later,
- He was still fleshing out the idea about Haki. The three colors, there interactions etc. etc.
- He saved them for the time skip region to show a clear way that our protagonist's could have a major power up. Ussop, Zoro and Sanji after Haki are a thing to be feared as learnt by Doflamingo.
A bit offtopic maybe but i have been skimming through some older chapters and i came across this during luffy second encounter with croco. When he was hit and grabbed by luffy he was briefly shocked and referring to something with "could he have"... Now assuming the translation from japanese is 100% correct and knowing crocs background now, i would say he was asking himself if luffy has awakened his haki ability. That is my interpretation based on the context however its only my interpretation.
Looking back at things now, this was maybe luffys biggest "ass pull" victory, he should have never been able to defeat him at that moment. (the chapter in question is 199)
You're missing some critical details.
Crocodile handed Luffy his very first huge loss. Luffy was at such a disadvantage against him in their first fight, through rive minutes of futile attacks. If it wasn't for Robin saving him, Luffy would have been dead from implement and being buried alive in the desert.
Second time, even when Luffy comes prepared with Crocodile's weakness, he still loses. Why? Because Crocodile was smart enough to train his power to counter its own weakness. The fact that he can absorb moisture from both humans and even the land around him to create his own personal desert just highlights how experienced he was. And once again, Luffy was brought to the brink of death.
Do you see where I'm going with this? It wasn't that Crocodile was weak, but for the purposes of the story, he couldn't keep defeating the main character over and over. The fact that it took so many times to actually succeed in the fight just goes to show how dangerous a threat he was.
It's not really hard to understand guys. Crocodile was just simply strong enough to be a Shichibukai/Warlord. You can't become one unless you demonstrate your power and influence in some way (the only exception is Buggy) and that includes Crocodile. Don't let Luffy beating him fool you, Crocodile was/is still a veteran whos leagues beyond him in skill/experience like any other Shichibukai (again, excluding Buggy lol). We simply place limits on Crocodiles capabilities because he lost to Luffy but the fact is he's never shown his full strength, especially against Luffy.
He has able to directly clash Doflamingo (resulting in a massive shockwave), tank a blow from Jozu, the physically strongest fighter on a Yonko crew, and recover immediately, he was able to party Mihawks blade and emerge from a battle with him unscathed (whereas Luffy could only Dodge and run away from him), and he's one of the most skilled DF users with one of the deadliest DFs and knowledge (and highly likely mastery) of DF Awakenings. All these feats are a cultivation of his physical abilities , skill and battle experience. The problem is a fairly simpleminded segment or the OP fanbase population have gotten so caught up in the increased usage of Haki in the New World chapters that they were misled to believe any and every Logia (minus the hyped Admirals) is helpless against Haki, even a highly trained Logia like Crocodile with techniques that can counter conventional Armament Haki defenses and decades spent in Paradise/New World.
Different people have different strengths guys. The level of effectiveness depends on its user, whether it's relying primarily/solely on Haki, DF or etc.