Are there any open-source military/war strategy simulating engines/frameworks?

Check out the Battlefield Simulator at CodeProject.

If you looking for military simulations then: sorry but this get complicated.

Delta3D Open Source 3D gaming & simulation engine used for military training simulations. It does include some vehicles and other 3D assets but not a huge amount, you can import 3dsmax8 .Max files.
You can add SimCore to get HLA support plus munitions modeling and apparently even weather system. Plus you need a RTI e.g. CERTI

The common military Transmission and Interchange Formats are HLA and DIS (and CIGI)
Open-DIS this is the Open source version of DIS, its mostly developed by US Navy and Airforce officers. They are currently considering using Project DarkStar for the distributed framework.

An example game where DIS is use is ACM which while its rather old it does include DIS support. and the Unix version can be downloaded from here

There are a number of DIS C++ libraries available e.g. KDIS

AFAIK DIS is still around but is largely superseded by HLA-RTI
You can find a list of OS and commercial RTIs on Wikipedia here

The US has an "Open Source" simulator called OneSAF but it's open source only to government and military developers only. :( You can however download some docs from the website. Old Screenshot from around 2004.