You win by finding the source of the alien threat and "deal with it" once and for all on a "final mission".
You can lose by several ways:
1) You were in debt for over $1,000,000 for two months. Your sponsors had had enough of your mismanagement and terminated the XCOM project. Earth was conquered by aliens not long after.
2) You had two consecutive "badly losing" months (big negative score, see next section), and your sponsors had had enough and terminated XCOM project. Just how bad is a "badly losing" month depends on your score and difficulty level (Scoring is explained in OSG Chapter 1 and next section).
3) You lost all of your bases to the aliens.
4) You failed to complete the "final mission" (see above).
*Alien Soldier > Alien Mission Information
*Alien Medic > Species Information
*Alien Engineer > UFO Blueprints
*Alien Leader > "The Martian Solution"
*Alien Commander > "Cydonia or Bust"
*Alien Corpse > Dissection of Species
I havent beaten the game yet either. I'm thinking that you would need to of researched an Alien Commander at the very least to get the final mission. ..and all things considering:
- a way to get there (Ultimate Craft/avenger?) - a way to resist the mind control for attaining live commander and/or base assaults (Psi investments in facilities and soldiers?) - weaponry to take out alien bases or Very large battleships
If you have a save point prior to this July 99 limit, maybe you can grab an alien commander then have it researched before july 99?