In Ender's game, right after the Second Invasion, the International Fleet's main secret strategy was
sending out ships to the Formic home world as soon as they had a working ansible.
However, how did they know where that was? Remember, it was dozens of light years away, and even if they had the general direction, when traveling such distances across galaxies you can't just guess and hope you get the exact angles you'd need.
From chapter 13 of Ender's game: relevant point in bold
“What’s all the secrecy for, anyway?” “Because we've taken some terrible risks, Ender, and we don’t want to have every net on earth second-guessing those decisions. You see, as soon as we had a working ansible, we tucked it into our best starships and launched them to attack the buggers home systems.” “Do we know where they are?” “Yes.” “So we're not waiting for the Third Invasion.” “We are the Third Invasion.”