History professors generally don't expect students to cite "common knowledge." For example, if you write "Battle of Gettysburg was fought in July 1863," you don't need to add a citation to some reference backing that up. It's a widely known fact. More specific forms of information need to be cited. To go back to Gettysburg, if you were to include statistics about, say, the breakdown of injury types at Gettysburg, you would have to cite those. So, the sentences that you cite are ones in which you are referencing specific information, ideas, arguments, quotes, etc from a source must be cited.

Quoting from Princeton University's page on academic integrity:

Or if you use a piece of information discovered by another scholar in the course of their own research, you must cite your source. But if the fact or information is generally known and accepted—for example, that Woodrow Wilson served as president of both Princeton University and the United States, or that Avogadro’s number is 6.02 x 1023—you do not need to cite a source. Note that facts are different from ideas: facts may not need to be cited, whereas ideas must always be cited.

As you can imagine, history papers often do have lots of footnotes/endnotes. However, a paper that has a footnote after every sentence would probably be a poor paper, as the main point of writing essays is to offer your own interpretations and conclusions. You take information and arguments from other sources - which are cited - and analyze those things. You might reach the same conclusion that someone else has - your average undergrad history paper is not going to contain some sort of groundbreaking thesis - but if you came to that conclusion on your own and can show your reasoning, that's ok.

In any academic setting, you should be presenting your own interpretation of the topic presented to you, and the assumption is that you're not going to bodily lift entire passages instead of actually thinking about the topic.

For example, if I were asked about, say, Continental influence in early medieval Scandinavian material culture, I could talk about how similar Frankish winged spears are to Scandinavian lugged spears, and riff on that. I would need to cite archaeological or museum journals when talking about the physical ways the spears were designed, and how they were constructed; I would cite textual sources describing their usage; I would also cite archaeological and textual evidence of other material influence from the same general period, but I should be coming up with my own interpretations of all of those sources. If my interpretation is similar to others that I've not read or I'm not aware of, that's fine - it means that I'm on the right track as someone else. Or we're both grotesquely wrong, but for the same reasons.

In addition to the other comments, I want to point out that it is sometimes appropriate to give a citation for a whole paragraph rather than each sentence. If I were using several sentences to summarize and article, especially if I didn't want to make use of quotations, I might cite it at the end of my summary as the whole article, rather than citing each sentence that contributed to my summary.

When dicussing history, you'd be in pretty poor form to simply list loads of facts about any given event, and you'd probably struggle to get much higher than a C+. In essays, you will usually cite facts (e.g. troop compositions etc), but the way you use those facts is usually the entire point of the essay.

For example, for my dissertation I needed to interpret Wyatt's Rebellion in 1554. In order to do so, I researched some general facts (such as the current makeup of the English cabinet) and then looked at more specific information in the form of primary sources (such as diaries from contemporary soldiers to determine morale, or Calender State Papers).

I would then give my own interpretation of events by saying that it wasn't a crisis for the English Monarchy, and explain why (through my research) I believed this to be true and why other interpretations by other historians might not be true.

So essentially- When it comes to facts, you probably need to reference it unless it's just something you know, but the bulk of what you're writing needs to be your own interpretation of events and why other historians have got it wrong

And as others have said, I've found that it's preferable to simply reference at the end of each paragraph rather than each sentence, unless you actually used more than one book/article or if there's multiple facts/figures from different sources. Referencing the same book twice in two successive sentences just bloats the page imo but that's just personal preference.

I think that it might help you to consider how history is written. Please keep in mind that this is a simplification for the sake of an understandable explanation. While history as an academic discipline doesn't exclusively revolve around written sources, they are an essential part of how we write history. Most of what we know about history, is based upon the written records left behind by those who came before us. This can range from records of debt to official law documents. These are considered primary sources. They are the evidence of what happened in the past if you will.

Now, whenever someone analyzes said primary sources to construct a narrative, they create a secondary source. While this is obviously not always a clear-cut distinction, this is the basic divide and might help you understand the process. Historians typically use both primary and secondary sources to write an academic paper. They will take all the information they find relevant and attempt to work it into a convincing argument. So for example, a historian who wants to research the socio-economic restrictiveness of guilds in medieval Paris will comb through the documents left behind by these guilds to look for hefty participation fees or similar limitations put in place to restrict participation. They will also use research done by other historians to give proper context both historiographical as in the historical reality they are describing.

They will then write their analysis of this perceived historical reality based upon this information. So while their work is based on sources, which should be quoted, a big portion of their work is their own analysis and the patterns they discerned. That's why you don't end up quoting all the time. Ultimately, writing history isn't about listing facts, it's about use available information to analyze and discern patterns in order to form your own theory or hypothesis.

You might be thinking : "Wait, I'm supposed to use primary sources?". You don't have to. You'll be basing yourself on the work of historians who have based their work on primary sources or reliable secondary sources. You are basically building your paper upon their research. That's perfectly acceptable for any paper written outside of academic circles. It's even possible for historians to base their research entirely on secondary sources. However, this generally only works well if these sources are respected works by qualified historians or if the topic is more theoretical in nature.

This also works because some well-known research done on primary sources is so methodically pure that it's basically accepted as fact through academic concensus. That doesn't mean that you can't reexamine the original research in an attempt to change the academic concensus, but it does mean that this knowledge isn't considered contended information, can often be found without citations and that it can be used to base your research on. This is why the denial of the holocaust is ultimately considered to be anti-historical. There are certain facts we have to accept as a basis of our shared reality. Whatever the academic discipline, knowledge is ultimately based upon the acceptance of certain things as a factual reality through academic concensus.

This is why it's not "unacademic" or "unscientific" to not cite common knowledge, as /u/eightgear has pointed out.