The scientists using dangerous chemicals prepared the virus after three months. After that, all aliens got ready for the war. The problem was that the virus couldn't be released directly into the Earth's atmosphere from Mars.
Moreover, a large number of viruses were required to affect billions of humans. Aliens' population was just a million at that time. Half a million aliens were needed in the process.
So, half members from all families joined the war. In half a million UFOs cabinets, the virus was kept.
They attacked humans suddenly at night without even sending a signal to the Earth. Just after they showed their presence there and tried to release the virus, they got shocked. The thing happened that was never going to be expected.
The virus from every cabinet disappeared. Aliens were searching for that when the humans came with their weapons and attacked them. It was unimaginable that all of them were killed. Half a million UFOs reached Mars with dead alien bodies.