You manage to unite the other nations and have 6 months to plan a defense.
Let's say that the Empire gets through via a wormhole, and, not wanting to risk their entire navy, sends 100 imperial 2 class star destroyers and their typical escort ships and starfighters with them. They have as much supplies as they can fit in that many ships.
Your forces:
You have access to 50 planets spread out over 10 star systems. Of them, 10% account for 50% of your total industrial capability. The separate star systems are interdependent. Your industrial capability is that you can produce your era's technology 100 times faster than modern day industries can produce their era's technology.
You have access to 100 billion civilians. 20 billion of them are fit for duty. 2 billion are willing to volunteer. To train 20 billion civilians with your capabilities would take a year.
You have access to 30 million soldiers, 100 million reserves and 300 million local militias. Your soldiers have different specializations matching modern day specializations.
You have access to 100 of your equivalence of a star destroyer and 500 frigates.
Each planet has 100 ground cannons as powerful as your equivalence of a star destroyer.
FTL travel is not available; fastest transport ships go at around 99.99% the speed of light with some sort of way to prevent the time dilation using some sort of hand-wavium technology. FTL communication is, however, and long distance communication speeds are comparable to Star Wars hyperdrive speeds.
Political stability is pretty high. Democratic, but for this scenario you are granted emergency powers.
Standard soldier:
Armor has sensors, radio, nanotechnology healing

Weapons are upgraded from modern technology; still relying mostly on projectile weaponry but with accelerated speeds, a pistol round would go at, say, 10,000 mph. Laser weapons available with power ranging from 500 gigawatts (pistol) to petawatts (mounted on vehicles).
Training varies, some being rookies and others being very well trained.
Armored vehicles:
Machine guns and cannons use almost relativistic speeds; a tank cannon round might go at, say, 20,000 mph. Armor can withstand said weapons at about the same rate as our armor can withstand out weapons. Lasers are also available, with power generation in the petawatts.
Airplanes:
Speed around 5000 mph, similar maneuverability to modern day planes when moving at that speed, uses weapons and armor similar to those on armored vehicles. Occasionally used in space.
"Star destroyers" equivalence:
Comparable in size to Star Wars star destroyers. Relativistic cannons of 500 kilograms going at 50% the speed of light, about 10 are mounted on your equivalence of a star destroyer. About 50 high energy lasers with about an exowatt of power are mounted as well. Hundreds of missile launchers, each with a yield of about 500 megatons, are mounted too. Armor involves some sort of semi-handwavium AB matter technology or nanotechnology armor or something like that, can somewhat withstand a 1 gigaton explosion. 500 point defense lasers can shoot down incoming projectiles and fighters with a power of about 10 pecawatts. Targeting system accurate to 5 meters in 100 kilometers. Combat range ranges from millions of kilometers for the missiles (which are homing, and the missiles and other projectiles technically won't stop until they hit something since space has no friction) to a few hundred thousand kilometers for the relativistic cannons. Can typically hold about 10,000 soldiers.
Frigates:
More maneuverable, faster and cheaper than the star destroyer equivalences, but 10 times slower and with 10 times less weapons. Similar armor time but less thick.
Did I miss anything?
Can you succeed? What's your strategy? In this scenario fending off the invasion force constitutes as a victory; the Empire collapses via ROTJ by the time the war is over.