Captain America #700 is the culmination of and overarching story that started at the onset of the Marvel Legacy relaunch. After the public's trust in the red, white and blue Avenger had been broken thanks to the horrific events of Secret Empire, Steve Rogers set out on a cross-country tour of the United States, to connect with the little people and save lives along the way.
For three issues, Steve rode his bike from state to state, but his trip was cut short when he was once again frozen in ice, this time at the hands of a new villainous organization known as Rampart. Steve woke back up in the year 2025, a future where most of America had been decimated by a nuclear attack, and the remnants of the once-great country now ruled by the grotesque King Babbington and Rampart.
The next two issues chronicled Steve's fight for the soul of America, against the overwhelming forces of a broken world. He may have ended up defeating Babbington, but issue #700 shows that Cap still has quite a fight ahead of him. Now the leader (nay, King) of America, Steve finds himself dispatching troops all over the country to fend off would be invaders, all while combating famine and dwindling natural resources.
Finally, after almost a year of fighting a losing battle, Steve recognizes that perhaps the only way for him to win is to return to his own time, to stop Rampart before they could even launch their devastating nuclear attack. Thanks to the help of Bruce Banner, Steve manages to travel back to the present day Marvel Universe -- more specifically, he finds himself back in the events of issue #697. Back in the present, this now-future version of Captain America sees his younger/present self get frozen before his very eyes.
From here, it becomes a race against the clock to stop Rampart from launching their nuclear missile. Steve finds himself aboard the villainous group's submarine and there, he latches himself onto the missile. Before it can lift off, he puts it all on the line. As Steve nears his goal, it becomes clear that he is on a suicide mission.
Before long, we see that this moment is meant to mirror the ordeal that Steve went through when he was first frozen, all those years ago, back in World War II. Once again, he finds himself hanging onto a missile, risking his life to prevent it from killing millions. Once again, with hope in his heart, Steve presses on, and manages to destroy the missile in a catastrophic explosion that takes out the submarine. Only this time, the blast kills him in the process.
Yes, Captain America dies in order to save the free world. But, in the water, the Steve Rogers who had been frozen only a few minutes before breaks free from the ice, amid the wreckage of the flaming submarine. Therefore, the Captain America who woke up in the future, the one who fought Rampart and King Babbington and fell in love with a woman named Liang, who we have gotten to know for the past three issues, is gone. He became an alternate version of Captain America, and his sacrifice allowed his younger, current self to continue his travels across the country, without ever having gone through a detour in the year 2025.
Captain America #600 revealed that Steve Rogers was alive. One hundred issues later, Captain America #700 gives us the death of Steve Rogers. In life, or in death, in sacrifice or triumph, the character proves himself timeless, and the best version of oneself one can hope to be. At the end of the day, whether through others or himself, Captain America lives.