Forged Alliance is a stand-alone addon for Supreme Commander. Basically it is an independent game, but continues the storyline of Supreme Commander: Black Sun has been activated. This activation also made it possible for the Seraphim, who had long been considered extinct, to open a quantum fissure through which they completely bombed Earth in a devastating first strike. Out of necessity, the three factions of the predecessor, Aeon, Cybran and UEF, work together in a joint alliance against the Seraphim.
For the multiplayer this means on the one hand another faction that you can play. On the other hand there are a few new units. FA is therefore to be preferred for the multiplayer.
There are two major version types, the Steam version (2019-02-09: current version 1.6.6) and the DVD version (latest version is 1.5.3599). According to several internet sources, you can also download the game with the CD key of the DVD version from Steam, but then with the newer version 1.6.6. Both versions are savegame compatible with (you can load and play the Steam version with the DVD version and the other way around), but they do not allow a shared network game (connection fails). Likewise, they are not replay-compatible (in either direction).
Essentially the usual way: Install from DVD, patch, if necessary copy a crack over it. Alternatively, run the Steam installation. The DVD game can also be started from a copied folder if you have already backed it up or from a previous Windows installation. The patch 1.5.3596 to 1.5.3599 cannot be applied anymore, even if you specify the directory where FA is located. The patch also doesn't work with the cracked version 1.5.3596. Version 1.5.3599 doesn't seem to contain a DVD query anymore. A crack is therefore no longer necessary with this version. Therefore, the recommended procedure for the DVD version is to install the game including the patch and to save it if you don't want to install it again later.
Both versions are playable up to and including Win 10.
The game mechanics follows the Total Annihilation Mechanics. Compared to Supreme Commander the economy is somewhat faster. The units themselves are just as fast, only the construction is faster. Some units are new (e.g. automated construction buildings). However, not much has changed in the core gameplay.
Some might have the impression, that the AI never uses nukes. That is not true! They actually do, but usually not earlier than one hour into the game. They are then thrown at the commander. But not all of them do. Current situation looks like this:
The last two require some further testing. As written above, the AI needs some time to get to the point, where it throws strategic missiles at you.
The game can be played in several ways over the network. The Steam version has an extra Internet multiplayer lobby that replaces the GPGNet lobby. (GPGNet is offline now.) Via the LAN you can connect with direct IP and port input. The game allows you to either choose the port randomly when opening a server, or to set it yourself. What doesn't work is player with the steam version and DVD version in one game. Both sides can't see each other or connect to each other. The game doesn't give an error message, but the connection just doesn't work. Similarly, a connection via the Internet (without VPN) does not work, despite port forwarding.
For a network game, FA requires the specified port and 15000 (UDP) to be allowed in a possible firewall.