The Blackhawks were an international paramilitary group of pilots operating against the Axis powers during World War II.
The Blackhawks came from the Nazi-occupied nations of Europe; Poland, Norway, Holland, and France. These men grew tired of sitting by the sidelines and rose up to free Europe from Nazi oppression.
When the Justice League traveled back in time to thwart Vandal Savage's interference with the historical events of World War II, they teamed up with the Blackhawks to destroy the Nazi's artificially advanced factories and their American invasion force after it was forced to turn back to Europe.
After the war, the Blackhawks stashed a number of "sci-fi" objects on Blackhawk island and had it guarded with machines.
Among their known trademarks are their dark blue uniforms (Blackhawk himself also sports the unit's logo as a chest symbol), and their battle cry, "Hawka!"
The Blackhawk Squadron first appeared in Military Comics #1, a Quality Comics title. They were fighter pilots who fought to end tyranny everywhere. At the start of the Silver Age, Quality was bought by DC, and as one of the few titles, Blackhawk survived.
In the 1970s, the title was less popular, as there wasn't really a need for the Blackhawks, who had by now become a team of adventurers like the Challengers of the Unknown, the Sea Devils and Cave Carson. They were eventually changed into a team of super-heroes, which doomed the series.
Mark Evanier brought the series back in 1982, rebooting it as a period piece, firmly establishing it as a World War II team only. In 1988, Howard Chaykin did another reboot, grittier and more politically loaded. To confuse matters even worse, the time travel reboot Zero Hour transported Zinda Blake, Lady Blackhawk, (and most of her history) to the present day. Zinda had not been part of either Evanier's nor Chaykin's reboot. The team that has appeared since Zero Hour is a mix of the original and both reboots, but is not really consistent. Currently, the only ones still alive are Zinda Blake and Janos Prohaska, Blackhawk himself, though he is wheelchair bound and according to the records, long dead and buried.
Similarly, the DCAU Blackhawks feature elements of the entire history. Chuck Sirianni and Mairzey appeared only in Chaykin's stories. Blackhawk is a Pole, which also comes from Chaykin; he was an American, or Polish American, originally. Olaf is Norwegian (original; with Chaykin, he was Danish). The items in the Blackhawk museum all come from the original era too; Chop-Chop's coolie uniform and Zinda Blake did not appear with Evanier or Chaykin.