I can't quit the Mission!

Hint 1 of 2:

Press the pause button (P). Then, there's a quit button.

Hint 2 of 2:

Press CONTROL-D 4 times to destroy your agents. When the message "mission
failed" at top of the screen appears, press the space button. You can replay
the Mission directly this way without going back to the main menu. An
alternative for this is the R button.