Here's the brief description of the main plot for Gundam SEED.

In the cosmic era, a war between the earth forces and ZAFT rages on. They fight with mobile suits known as GUNDAMs. In the midst of all this, two young people(the main characters) Kira nad Athrun who were once friends are forced to face each other in battle. The objective? To kill each other.

How did this war start?

A while ago, a new 'breed' of humans were created called coordinators. These humans had special abilities that the naturals(normal people) did not have by enhancing their genes before birth. Soon the naturals grew jealous and saw the coordinators as a threat. The result? The descruction of Junius 7, a neutral planet for the coordinators. So thus, the war started.

Very brief summary isn't it? But's it's the main plot, lots of unexpected twists happen and expected events don't occur. Gundam SEED is definetly a very emotional story about war and loss.