D-Day is a FREE game designed for PC, and is also playable on Mac and Linux. You can play as 7 different World War II armies: USA, Germany, Japan, Britain, Italy, Russia, and Poland! It has a ton of maps and can be played in multiplayer or single player against bots. It's an old game (originally made in 1999) but it is a lot of fun and has a lot of loyal fans. And you can play it on an older computer. Check it out.
Here is the full download which has everything you need to get started. It is 206.9 MB, hosted on mega.nz:
Look at the servers link on the left to get server addresses to play online and see if anyone is playing. Most servers have bots so even if a server is empty you can play and have fun until someone else joins. Feel free to say hi on the forums or ask for help there if you need it. Also there is a discord link on the left that has active players you can chat with.