I've been behind on email, so this may be redundant, but there were
lots of pre-20th century and for that matter pre-gunpowder leagues.

Consider the various Greek alliances against Persia, and later
Macedonia, and still later Rome (weren't always successful).

Swiss history could also be used. When the Sempach Covenant of 1393
allied 8 cantons they had been fighting the Hapsburgs for a century,
defeating armies of the biggest monarchy in Europe each generation.
There was no regular assembly, no assembly, no executive, no capitol,
no treasury, no chancery, no registry of documents, not even a seal or
flag. Also at that time there were three more associates, the Gray
League, the League of the House of God and the League of the Ten
Jurisdictions. All that (barely) untied them was a determination for
local rule and hatred of the Hapsburg.
"The Swiss cantons fought for republican liberty and feudal
independence; but they also fought their neighbors and fought each
other, and they had as yet no state. The Greeks would have

I recommend getting THE END OF KINGS by William Everdell, a history of
republics and republicans from the prophet Samuel and the political
Solon and down to the early 1980's.

The area of the proposed alliance was all former provinces of the
Empire. So they have a common language, history, folk tales, religion
and culture ... not yet diverged into separate languages or nations.
Loose alliances of kinfolk against a common enemy (even when there are
murderous wars with neighbors before, after, and sometimes even during
the alliance) have been going on for several thousand years.

Lyndon the (buried by homework) Lurker