I know you are still upset at losing us, but now you must put aside your hard feelings for the common good, in case it has escaped your notice but a very powerful foreign enemy is currently invading our planet, we must stand together in order to repel this enemy, or else we fall, like dominoes, one by one.
—President of the Confederacy Jane Zarkan appealing to the less eager members of the Union Senate to ally with the Confederacy
The Confederate-Union Alliance of 18 BBY was a short-lived alliance between the Confederate States of Earth and the Union of Earth States, the alliance was born out of the Imperial invasion of Earth in 18 BBY, the Confederacy called on the Union to put aside their differences to unite against the common enemy.
The alliance only lasted mere weeks before the Union of Earth States fell to the Empire and was reorganized as the Earth Empire puppet state, resulting in the alliance being annulled and the Confederate forces withdrawing from former Union territories in order to defend their own nation, the Confederacy fell several months later.
Background[]
From traitors to allies? Unthinkable!
—A Union senator opposed to the idea of an alliance with the Confederacy
The Confederate States of Earth was original founded by 10 nations of Earth who broke away from the Union of Earth States in 21 BBY, the Confederacy allied with the Confederacy of Independent Systems, resulting in the Galactic Republic siding with the Union.
At the behest of the Republic a joint Union-Republic invasion force launched an unprovoked attack on the Confederacy, resulting in the widespread condemnation from citizens of the Union, during the Union offensive war crimes like the Burning of Valley City and the Sacking of New Jackton were committed, which further inflamed condemnation from the Union people. As a result, many nations seceded from the Union and joined the Confederacy, resulting in the Confederacy consisting of 95 nations, with the Union consisting of only consisting of 15.
The Republic was defeated in the First Battle of Earth, and fled Earth, effectively leaving the Union to fend for itself against a much stronger, and rightfully angry, enemy, the Union lost a lot of respect for the Republic.
The Confederacy won the Earth Civil War of 21 BBY - 20 BBY, the Treaty of Williamstown guaranteed Union respect for the Confederacy's legitimacy in exchange for being allowed to remain a completely independent nation.