Assault Squad is a Multiplayer expansion, although there are 16 Skirmish missions you can play single player, they are designed for Co Op play. There is no campaign.
The multiplayer aspect of the game has had a lot of attention, balancing the factions and fully incoporating the Japanese who are great fun to play with. Highlights are the mortar truck and kamikaze infantry. Frontlines has been streamlined and is now more fun although it is sometimes difficult to get a game started as Assault Zones is far more popular. In the latest patch (#2) DMS have added ranked servers and also a few more units including the Stug III. Each patch they have updated the tank models to high poly and resolution. The new Panther, Tiger and Shermans look awesome.
The Skirmish missions are set factions, so there are 3 for each side where you battle a pre set defending faction. This is not how alot of people percieve Skirmish, so for example one Mission is Battle of the Bulge, you are the USA and the AI is Germany. This can only be changed through a potential mod. Also the Skirmish missions dont give you access to the full range of units, however there are already mods changing this. Check out the DMS forums. There are 4 difficulty settings and if you are new to MoW, I strongly recommend starting on Easy. Even Normal can be tough to crack on your own. Hard should only be attempted with more seasoned players who work together and communicate. Mumble is a great piece of voice software which greatly aids Co Op gameplay. Overall whilst the Skirmish missions are similar in layout and units available, they offer hours of fun and Co Op is much more rewarding than playing on your own. On Hard 1 skirmish mission can take 2 hours + to complete. Heroic is another story.
Overall the support for Assault Squad since release has been amazing, adding new units, listening to the community feedback and really refining the online experience.
It is a shame that the frontend of the Multiplayer is not as user friendly as much of the community would like.
Its a tough game to master, but when you do it is a WW2 strategy experience like no other.
Keep a look out for Downloadable Content adding new Skirimish Missons, hopefully some defence based and also adding more units to the fray.