I just got a Gamersgate newsletter which explained the situation.
If you pre order from Gamersgate, you will get the Spanish American War scenario, which is described as the first DLC, at no extra charge.
This scenario will also be available for purchase by itself, for those who do not get the bonus.
Since this is described as the first DLC, it looks like there will be other scenarios sold as DLC in the future.
I do not know if you will get the bonus scenario if you pre order from anyone other than Gamersgate.