It's very much possible to as a historian focus on several things. However, one thing that'll probably limit you in certain ways, is knowledge of languages. For a really professional specialization in certain areas or fields, it will be necessary to speak an at least one language but preferably more in which your primary sources are written. This is also in many cases a prerequisite for getting into various PhD programs. Where I am from you need at least a working knowledge of two languages other than German (native language) to get into an MA program. Now, for most people that is English, which everybody speaks here, and French, which is widely taught in schools. But when moving on into the area or topic of specialization, working knowledge of one of the languages your primary sources are written in is absolute key.

Say, you want to study the relationship between Marxist/Socialist movements and Zionism. Depending if you want to focus on the early 20th century or at a time when the Soviet Union already existed, for that it would be prudent to have a working knowledge of Yiddish, Russian, and preferably either Polish or German since those were prime languages in the continental European Zionist movement.

If you want to focus on the Soviet Union's role in the Israeli/Palestinian conflict, Arabic, Russian, and modern Hebrew would be the way to go but either modern Hebrew and Russian or Arabic and Russian would be very advisable since without them you' severely limit your perspective to only one of the historical actors in this case. That too would be doable but then you'd have to be open about limiting yourself to how Israel perceived the Soviet Union e.g.

Generally speaking, there is no limit on how long a timeframe historians can specialize in. People like Braudel e.g. have pioneered an approach of long durée and have dealt with hundreds of years of development. The reason why many programs focus on modern Europe, or Jewish history or Middle Eastern studies etc. is one of language and methodology.

As a historian of modern Europe, I am generally familiar with what medievalists e.g. do but I am certainly not qualified to start doing medieval history in more than the very broadest sense since they rely on different sources, sometimes different methodology, and in terms of languages I lack the distinct skill set to read, let's say, German documents from the 13th century and I am only superficially familiar with the current concepts used in the study of these documents.

In your example, you are within the confines of modern history, spanning from Europe to the Middle East and the US (I'm leaving China out for now since that is once again, a different beast altogether in some ways). In the end, what is going to limit you in connecting all these, is a knowledge of language and your ability to only read so much about a certain topic.

There are several possibilities to combine this however. The study of socialist Zionism in a certain time frame. The study of Jews in the Soviet Union and other socialist countries. The history of socialist countries towards the Middle Eastern conflict. Marxism and Arab national movements and so and so forth.

As I said, realistically, the study of these topics is possible if at some point you are willing to invest the effort to gain a working knowledge of relevant languages because at some point you are going to want to see the primary sources for what you are doing. And even if that's impossible, there still is always the possibility to write about the relation between Marxism and Zionism resp. their perceptions in American Jewry.

So, multiple areas of specialty. This is a tricky question. Most historians, I would say, have TONS of areas of decent interest and several that they wish they could specialize in. You'll ever find some successful, career-academic historians, who shift gears over the course of their lifetimes/careers--starting off in one subfield, finishing with a magnum opus in another.

But for the purposes of PhD program admission, you need to be able to specify a single research project--i.e. what you think your dissertation will look like. (In the UK, it probably will; in the U.S., after two years of coursework, it probably won't).

That usually means being able to identify one range of subjects within a subfield--"Yiddish culture in America and Palestine" and then having some idea what your take on that might be, like generational divides or language morphology or fashion or whatever.

Some people do conduct comparative work! But understand that comparative work isn't just "studying two random things"--it's bringing two topics (even if they're "medieval Italian Church history" and "medieval English Church history") into comparison and contrast to say something new about both of them that isn't visible alone.

But comparative work is DOUBLE the work. Honestly. Because you (might) have to learn two sets of languages, as /u/commiespaceinvader says. You WILL have to learn two or even three sets of scholarship: all the scholarship on Topic A, all the scholarship on Topic B, and a good bit of scholarship on comparative methodology.

That's fine if you have all the time in the world. That is very, very difficult to do under the pressure of a ticking dissertation clock.

None of this means that you can't love all the things you love, or read about them in your spare time, or plan to conduct original research on them somewhere down the road. What it means is, you'll write a dissertation you can finish.

It sounds, based on your description of your historical interests, that you are generally passionate about the past, and now need to narrow down your interests to determine which part you are most passionate about. As problems go for potential future historians, this one is pretty good to have! I agree with u/commiespaceinvader 's comment that Jewish Studies seems to be the linkage between the majority of your interests. You ask if someone could study both the Middle East and Europe -- well, if they are studying a group of people who lived in both places, such as modern Jews, they certainly could.

I also agree with the advice offered by others on the question of languages-- that is going to be the biggest factor that will limit your future primary research. You say you are interested in Yiddish culture and radical leftism as it relates to Jews. In that case, I would suggest studying Yiddish (if you have not already). It was the language of working-class Jews, and the language around which much radical Jewish leftism was built, in both Eastern Europe and Mandate Palestine. Another Eastern European language would also be very helpful. Russian makes sense if you want to study Soviet Jews, Polish if you want to study twentieth-century Jewry outside of the USSR (as it had the single largest Jewish population in Europe after WWI). Modern Hebrew will be useful if you want to look at Israeli politics at all.

Basically, I can imagine a variety of ways the thematic and geographic interests you laid out could intersect as research projects. Zvi Gittelman has a book about the origins of the Jewish section of the Soviet Communist Party known as Yevsektsia (Jewish Nationality and Soviet Politics), but more research could be done on what happened to individual members of the Bund after that organization was forced to disband and was partially absorbed by the Communist Party. Or more research could be done on women's involvement in radical leftist Jewish politics! Transnational topics are hot right now (for good reason- they have long been overlooked), so something on the direct connections between Mandate Palestine and socialist/ communist/ zionist movements in Eastern Europe would be exciting to read. But all of these are merely suggestions. It seems to me that you are at a stage where you need to decide for yourself what questions excite you MOST, out of a variety of potentially interesting topics. Which historical questions or issues do you think about as you are falling asleep at night? What is the most interesting history book you have ever read? The most interesting primary source? I suspect that if you ask yourself these kind of questions, and give yourself time to reflect, you will start to narrow down your areas of interest. Jewish Studies will allow you to keep lots of doors open, touching on a variety of places and time periods through the lens of Jewish history.

Good luck!