Battleground Europe (various pricing, 14-day free trial) made its debut in 2001 as "World War Two Online." It followed a familiar pattern: Launched too soon due to financial concerns, it had constant hardware and software issues and lost momentum, unable to attract a large audience. Nonetheless, it has soldiered on (heh), and is still running, and trying to attract and retain new players.
Battleground Europe is designed to be relatively realistic simulation of WW2 combat, set on a half-scale map of much of Europe and featuring complex vehicle and gear damage models. Crouching, stealth, and choosing the right weapon for the job are all important. Those things make it interesting to play, at least for a while. Low population, sluggish gameplay (you can spend 10 minutes running across a landscape, then die from a single gunshot), dated graphics, and a preference for realism over fast action may be issues for some.
There's a 14-day free trial and then a monthly subscription at a variety of costs. Paid plans start at $18/month for the "non plan" rate (higher than any other MMO I am aware of) down to $13/month for the 1-year plan.
Note: The Download button takes you to the vendor's site, where you can download the latest version of the software.