Why does Mazer tell Ender, you're the last?

Earth's military command had eyes on the bug planet, showing a massive and apparently ”military” bug build-up. Which is clearly depicted in the film and thoroughly hinted in the book. Based on this intel, Earth came up with an estimate date of the bug attack that served as a dead-line. Earthlings never considered this as a defensive maneuver.

Is it more clear now? :)

Immediately After the defeat of the Buggers in the second invasion, the International Fleet launched every ship they had in a direct attack on the Bugger home world. The reason for the urgency was because the fleet was getting close to the Bugger home world and they needed a commander for the fleet.

Ender entered Battle School when he was six, and he destroyed the Bugger home world at age 10, so when he was recruited, the fleet was only 4 years away from reaching the Bugger home world.

I missed it the first time, but early in the movie Harrison Ford looks at a screen which shows a fleet on the move towards a planet with just under a month ticking away. I thought that was a Formic fleet, but it was the Earth fleet approaching Formic space. Harrison Ford had one month to get Ender into shape to command the fleet.