"My name is Fury, Captain Rogers. Nick Fury." The man stood in the shadows...or perhaps the shadows gathered close to him. "The Avengers need you. SHIELD needs you. There are threats out there, threats to America that only you can face."
Captain America smiled wearily. "How can I turn my country down?"
They took him to the future, then, with a time machine they claimed was confiscated from someone named 'Doctor Doom'. They set him up with his own room in the SHIELD helicarrier, and told him he'd be going on his first mission shortly. Within minutes, a SHIELD agent met him at the door.
"So, how about a little sparring session? You, me and Wolverine against another team. The practice will do you good."
Cap shrugged and grabbed his shield. "Sure. It'd probably be a good idea for me to get used to some of these strange new threats, Agent..."
"...um, right. Just show me the way to the gym."
A few minutes later, and he returned to his room, bruised and exhausted. His 'sparring partners' had tried to murder him with some sort of powered suit of armor that shot missiles and lasers. Luckily for him, Howard Stark's shield held up to the punishment, but he'd be feeling the pain for days even with the Super-Soldier Serum coursing through his bloodstream. He needed to rest, or he'd be no good for whatever mission Fury was planning for him--
There was another knock at the door. "Hello, sir, I'm Agent Deathbringer. I'm afraid that Doctor Strange and I need you for another sparring session."
"Um, I really think that I need a little R and R, can't you just--"
"Sorry, it's tournament season." He pressed a button, and the two of them were teleported back into the gym. This time, a ten-foot tall green monster pounded away at him for five solid minutes while his teammates picked off its allies. Cap couldn't help but notice that they kept hiding behind him whenever anyone attacked.
He staggered out of the gym, but he didn't even make it back to his bunk before 'Agent Play947325' dragged him back for another 'sparring session', this time with a woman named Storm as his partner. In the brief moments before a crazed telekinetic tried to drop a boulder on him the size of a Buick, he whispered to her, "Um...do we ever fight bad guys around here?"
"Sometimes," Storm whispered back. "To keep in practice between sparring sessions."
Thankfully, he didn't remember much after that. He woke up back in his room, his skull ringing and a lump the size of a hen's egg on his forehead. He tried to sit up, but the room swam unpleasantly around him and he sagged back onto his bunk.
There was a knock on the door. "Go away!" he shouted.
The door opened anyway. Cap realized he'd never actually seen any sign of a lock on it. "Hi," the SHIELD agent said. "I'm NickFury232, and we have a sparring session scheduled. Don't worry, Cap, it's just to keep your skills honed."
Cap tried to hit him, but he was too dizzy to do anything but swing wildly. "I understand your feelings, sir," the Agent said as he teleported them back to the gym, "but we need to use the best in these fights. And you are the best."
He wasn't kidding. As the other team teleported into the room, Cap saw that they had another Captain America right at the front. "How...?" he mouthed, almost too confused to speak.
The other him shrugged, his expression resigned. "I'm from fifteen minutes in your future. Agent Dudebr0 here used Doom's time machine to make sure he had me on his side." He sighed. "If it helps any, I remember making it quick."
Twenty-one days later, they finally stopped coming. Cap felt like he'd been through World War II...again. He tried his best to patch up his wounds, though. The new recruit, a man named Cable, was explaining to him that it all started up again in a couple of weeks. Captain America briefly wondered if the living legend of World War II could get away with deserting.