It has long been said that speech influences prose, that the way we speak is likely to affect the way we write. One vehicle for this influence is television and film; for students and fans of history, this includes documentaries. Perhaps in order to build up suspense (at least for audiences unfamiliar with the topic or event being narrated), documentaries routinely use a strange—and I believe inappropriate—tense for talking about history: it's called "the simple conditional."
It goes something like this: "Hitler's troops would march into Poland." One half expects the narrator to follow with this: "However, they ended up turning around because the Poles were so unwelcoming," or "if, and only if, Hitler paid them high enough wages."
In other words, as the name implies, conditional tenses (they come in "progressive" and "perfect" models, too) require a condition, an "if". Here's an example: "Hitler would even persecute non-Jewish Germans if they opposed him or failed to fit into his eugenics scheme."
Many history students are mimicking this approach in historical writing. Yet how logical is it to write about past events using a conditional tense? If the event happened, then it should be expressed in the (simple) past tense: "Hitler's troops marched into Poland." Whereas it is appropriate to use the present tense when discussing the works of authors—even those long dead—it is not so when writing about events in the past.
Did Hitler's troops march into Poland? Indeed they did, on September 1, 1939; it's done, finis. So let's not imply any conditions on events that have already occurred; when we write about the past, let's use the past (tense).