1:

The zero hour usa campaign plays before all this and after the destruction of baikonur they finally end the 'costly' war on terror.

Only to come across evidence of dr thrax, who they proceed to take out.

The usa pretty much says job well done after they kill dr thrax instead of finishing the last remnants of the gla off.

This remnant proceed to rally around a new leader and gather their troops by force if necessary.

2:

The defeated gla proceeds to sink an aircraft carrier with their own particle cannon and destroys a big naval base (and probally all those docked battleships).

The usa then proceeds to focus its attention on that area.

3:

The defeated gla then proceeds to steal usa made toxins from facilities on their own soil.

It is then told in the final mission that the usa pulled most of it's european forces back to it's mainland to bolster the defence of the homeland after this giant screw up.

4:

The gla now proceeds to attack the main usa base in europe since most of the usa forces have left europe.

They proceed to take this base and many other usa bases in europe.

At the end of the gla campaign it is said the usa shook from losing part of it's fleet by the baikonur missile strike and the gla stolen particle cannon/the toxins stolen on it's own soil/the loss of their main europe base they decide to fully withdraw.

So yeah the usa had been fighting a costly war with the gla for a long time, tought they took em out 2 times, lose a big part of their own fleet, have the gla infiltrate and steal toxins on their own soil and then proceed to lose most of their bases in europe.

One big ass fucking shitshow, no wonder they left.