Balmorra:
Obi Wan was in the command center when he found out that the Empire had destroyed the republic fleet over Balmorra. He ordered the trained Balmorran troops and clone soldiers to concentrate around the 36 planetary shield generators.
since the shield was much stronger than the one over Alderaan, a direct invasion would be near impossible. Grievous was told that if the invasion was unsuccessful, he should at least blockade the planet to prevent arms from reaching the remaining republic systems.
Alderaan:
In the city of Alder, Luminara
was meditating. The Empire hadn't attacked the planet in the last month so she felt free to let her guard down. Then Commander Gree contacted her in a transmission:
"sir," he said, "we detected a separatist landing ship moving towards our position".
"Then shoot it down." Replied Luminara
"But here's the thing, the landing ship is not landing near us, it's crashing towards us".
"Then we should order an evacuation and hope for the best".
The transmission ended.
At that moment, Luminara got another transmission from the command center of another city in Alderaan:
"General Luminara," said the Alderaanian commander, "there is a separatist landing ship heading on a collision course".
"Then this is not an accident," replied Luminara Unduli, "it is a deliberate kamakazi attack".
"How do you know?"
"because there's also one heading towards Aldera".
"we've already evacuated the city so hopefully these ships don't deal too much damage to the infrastructure".
"it would be wise if you would do the same".
"wait a minute. The ships are set to crash a few miles away from the city. They have readjusted their course".
"the ship over Aldera is beginning to float around a mile above Aldera and it has stopped moving at a collision course and is floating a little bit above..."
Boom
"Luminara are you there? General Unduli, do you copy?"
A nuclear explosion went off inside the ship and onliterated Aldera and the other also blew up shortly after.
Later that day, other major cities of Alderaan were leveled using the same method. The next day, the Empire launched a full scale assault on the Republic world.
Alderaan was now under Imperial occupation.
Elsewhere:
The Empire had taken most of the Republic's worlds but they still had to subjugate the remaining neutral worlds such as Tatooine. Malgus knew that many worlds were useless but didn't want to give the Republic anywhere to run.