The third aliyah of Parshas Matos discusses the war with Midian, while the third aliyah in Parshas Masei discussed the war of conquest with the nations of Canaan. Each of these can be seen as a paradigm of war. The war against Midian is in order to remove the ones who caused Bnei Yisroel to sin. Any spoils taken from this war need to be kashered and immersed in order to make them useable. However, in the wars of conquest, even forbidden items become permitted for use, as we conquer and reform the land in order to make it a suitable environment for us.