Step Back, Fantastic Four: The Winter Soldier Is Now Doctor Doom's True Nemesis
Warning! This post contains SPOILERS for Thunderbolts: Doomstike #1The Winter Soldier just found out what it's like to become a nemesis of Doctor Doom...and there's a major cost. Having taken over the world with all the nations' leaders bending the knee to his rule, Victor von Doom is seeking nothing less than absolute fealty in exchange for world peace (and some pretty appealing increases to basic human rights). That said, Marvel's heroes know that Doom can't be trusted, hence Bucky Barnes' decision to defy the planet's new emperor.
The Winter Soldier's rivalry with Doctor Doom didn't just come out of the blue. Instead, quite the opposite was true at the beginning of Thunderbolts: Doomstrike #1 by Jackson Lanzing, Collin Kelly, and Tommaso Bianchi. Referencing their recent past where Bucky and his Thunderbolts leveraged Doom into killing the Red Skull, the dark new Sorcerer Supreme initially wants to reward the Winter Soldier:
Having been inspired by Bucky, Doom recently murdered Baron Zemo as well, Hydra's other predominant leader as seen in Marvel's One World Under Doom #1, an act which helped cement Doom's new rule over the globe. In return, Doctor Doom wanted The Winter Soldier/Revolution to join his empire in a key role...one Bucky immediately refuses.
The Winter Soldier Rejected Doctor Doom's Invitation
Turning Them From Potential Allies To Bitter Enemies...
Wanting Bucky to lead his secret police as a new iteration of Thunderbolts carrying out his will, Doom saw the potential within The Winter Soldier to become his right hand. However, Bucky's refusal is something Doom can't condone, let alone Bucky's attempts to undermine his rule. Destroying Thunderbolts Mountain as well as the LMD of Valentina Allegra de Fontaine, Doom gave Bucky a week to decide whether he would serve him...or face the consequences.
Choosing instead to defy Doom, Bucky infiltrated the global emperor's sub-orbital space station where he had been keeping the world's nuclear missiles. Intending to sabotage the entire arsenal, Bucky realizes all too late that the station is a trap, one laid out by Doom himself as punishment for The Winter Soldier's defiance. Considering what Doom does next, one can imagine that Bucky now regrets ever thinking he could defy Doom (without having to pay the dark cost that inevitably comes with it).
The Winter Soldier Turned Doom Into A Major Nemesis
Now He's Paying The Price
While he intended to burn Doom's entire empire down with help from Black Widow and Songbird, Bucky is now suffering one of the worst consequences possible. Doom suddenly launches one of the nukes, leaving Bucky with not nearly enough time to disarm the missile before it hits Shelbyville, Indiana (Bucky's hometown). As if the attack and its death toll of over 20,000 lives weren't tragic enough, Doom pubically blames Bucky for the attack and uses it as justification for the creation of his own team of Thunderbolts, one led by a new Citizen V who's revealed to be the real Valentina.

Marvel Admits a Fantastic Four Member Could Easily Defeat Doctor Doom and No, It's Not Reed
The Fantastic Four might be struggling to keep up with Doctor Doom during his Marvel Universe takeover, but one former member doesn’t stand down.
Seeing as how this is only the first issue of this new tie-in series to One World Under Doom, it stands to reason that Bucky will likely keep fighting Doom. However, it's quite remarkable that Bucky has already been dealt such a major blow mere days after becoming one of Doom's key rivals, especially considering he could have been one of the new regime's most powerful leaders. Had Bucky accepted, perhaps he could have worked to undermine Doctor Doom from within.
Thunderbolts: Doomstrike #1 is on sale now from Marvel Comics.