Well, they're one of the historic alliances of P&W that are still around, the other 2 being Rose and TKR. You also have the likes of Guardian, but they're not as large, and are usually low profile, not necessarily making power plays, but being involved in pretty much every global war nontheless.
Essentially t$ became the most influential alliance in the game after a war called Oktoberfest, where UPN were their biggest rivals. From then on, Syndisphere became the dominating sphere in the game, even though their enemies usually outnumbered them. t$, Mensa, TKR, BK and Guardian were all in the same sphere, and all of them were good at war.
Then TKR outgrew t$ and became the main powerhouse in the game till Knightfall, where the psheres looked a bit different. Afterwards we had NPOLT, which was the war where NPO tried to cheat their way into winning the game, failing miserably. Since then, the aforementioned historic powerhouses (Rose, t$ and TKR) continued being the most influential alliances in the game, although t$ went through a rough patch pretty much till their current leader Tarroc took over.
Finally, during the past few years, Eclipse emerged as one of the main alliances in the game, consistently being the most successful big alliance, having only lost 1 war so far.