From my knowledge, the entire war started when UFS failed to effectively address a particularly cold winter, and a lot of people died. There were other reasons the coalition formed, but that's a big one. Another one is that the United Federal States was effectively militarizing the entire country, and enacting laws that the people despised openly, but, again, nothing was really done about it, partially because of mass Stagflation. The Pacific states were neutral until a UFS raid on a "Coalition Training installation" resulted in the death of a large amount of PSA civilians. That's when the Pacific states got involved militarily, and they've been on the offensive ever since, taking the fight to UFS through highly aggressive tactics and strategies. Through CoST and PSA's eyes, the UFS government is corrupt and should be overthrown. Through the eyes of UFS, there is a lot on their plate, and they were ill-equipped to effectively handle the situations in question, but they also believe that CoST is effectively a well-equipped terrorist cell causing unnecessary instability and violence. Sierra Dynamics is a very new faction, so it doesn't have much of a storyline yet, but from what I've gathered they are a private military that provide weapons, vehicles, and equipment to the federal armed forces.
Weapon of Choice- KWA CQR Mod-1
"The most important thing in the Olympics is not to win, but to take part, just as the most important thing in life is not the triumph but the struggle. The essential thing is not to have conquered but to have fought well" -Pierre de Coubertin, founder of the Olympic Games