It started all the way back in 1908. When a rift in the fabric of reality caused a massive explosion that we later named the Tunguska Event. At first, we thought this might have been a one-time event. However, the rift let loose several unknown/theoretical elements into our reality. But it wasn't until three decades later when we saw the results of Tunguska.
The first instance involved the first wave of Masks (or as they're more commonly called, superheroes). And the first hero was Solar Woman, who's power over light and energy made her one of the most powerful masks in the world. More and more Masks keep coming out with unique powers of their own. And even if they didn't have powers, they took up a gimmick or used advanced skills to fight crime.
Of course, while there were plenty of good Samaritans using their gifts to help humanity, there were also villains who were using their gifts to hurt others and benefit themselves without care. And the Masks would do battle with them.
Then came the 2nd World War. At first, many politicians in the Allied Nations were debating whether or not they should use Masks in the war since the Axis had no problem using villains of their own. However, that discussion was scrapped when humanity meet the first other intelligent race in the universe. The Takara.
The rift was caused by one of their dimensional experiments and had only just learned of the effects it caused on humanity. As such, they decreed that Earth was to become part of the Takaran Imperium, regardless of humanity's objections. But humankind is a stubborn race, if anything else. So naturally, we fought back.
A defense initiative was spear-headed by the Allied Forces. Masks, scientists and military personnel from all the allied nations joined forces and succeeded in repelling the alien invasion. Afterwards, when the Allied Leaders formed the United Nations, they decided that a permanent defense force should be established in case another world-ending event would happen. And with the U.N. controlling it, this task force won't be under the control of a singular nation.
This was the beginning of the greatest alliance of Heroes, Scientists, Rescue Workers and Military Personnel. A task force that would save the world countless times and inspire new generations for years. These were...The Sentinels!
Author's Note: Welcome, my friends, to a celebration of all the great things about the Superhero genre! Believe it or not, the inspiration for this story came from Rob Liefled of all people! After seeing that he was back at Image Comics with the stupidly named Bloodstrike, I thought back to his magnum opus Youngblood and how it failed to be 'the next generation of heroes.'
However, as much as that book failed, it did give me an idea for a superhero series of my own to showcase everything great about my favorite genre. You might consider this a ripoff or improvement of Youngblood, but as Joel of the ComicBookCast once said: 'There are no bad characters, just bad writers.'
Also, I won't create twelve other generic teams so there's that.