The end of the age even has the famous 'Last Alliance' which confronts Sauron and cuts the Ring from his finger. However, the Second Age actually features several major unions and collaborations between the races of Elves, Men, and Dwarves, in particular. The Third Age is largely defined by the Free Peoples of Middle-earth fighting Sauron in isolated pockets.
While it's almost certain that the Rings of Power are the 'great powers' being discussed here, it should also be pointed out that it could be talking about something else: alliances. They're all initially owned by Elves, who are able to create most of the magic baubles with the help of their new friend Sauron - yes, friend. Of course, when they're originally forged, they aren't divvied up in that way yet. This includes nine for the Men that eventually become the Ringwraiths, seven for the Dwarves, three for the Elves, and one for the Dark Lord.
All of the major Rings of Power were forged roughly halfway through the Second Age.
