The best way to deal with this, I think is to use primary sources, to students what people of the time were actually thinking. Careful and repeated use of primary sources that run counter to students' assumptions can, eventually, cause them to think differently. Stephen Lazar, over on his blog, has talked about how using carefully paired, and therefore conflicting, primary sources can get students thinking.
There are two challenges, unfortunately, and both involve time. One is finding the time to actually find the good sources. And the other is to find the time for the repetition necessary to break down students misconceptions. One source, and one day, any not do it. But if my curriculum forces me to cover the outbreak of the war in one day, I may not have time to do all I need to do. But it's worth reflecting on this goal, and trying to make use of primary sources in this way more of a priority.