Question on wa placement in this example

In doing a recap of all my lessons to date, I'm finding much is clicking in place.  For the most part, even "wa" and "o" are starting to intuitively make sense most of the time.  I say intuitively, because most of the time I just seem to know where they go through lesson practice.  But I'm a little confused by the following:

In saying...

にほん じん です か？
Nihon jin desu ka? / Are you Japanese?

Why would we not say:

にほん じん は です か？
Nihon jin wa desu ka? / Are you Japanese?

The topic is whether someone is Japanese.  I'm not understanding why は would be omitted here.  

Thank you very much in advance...

は marks the topic, and I understand in this sentence the topic is the unvoiced/unwritten "You".

As English speakers I think we would be inclined to say/write, 「あなたは日本人ですか？」but the 「you/あなた」is unnecessary when speaking/writing Japanese (because of context), and so we end up with 「日本人ですか？」 

Hi イリニさん!

As 夫婦茶碗 said, 「は」marks the topic of the sentence, which in this case is the unspoken/not written "You".

If we put the topic in, the sentence becomes「あなた　は　にほんじん　です　か？」.

As 夫婦茶碗さん also said, in English, we usually make the topic clear in every sentence. However, in Japanese, it is not necessary to state the topic in each sentence. The topic is and can be usually understood from the context of the conversation. (Although, miscommunications can happen because of this, even among native Japanese people!)

Hope that helps! Please don't hesitate to ask if you have anymore questions.

Crystal

Ok, thank you both.

So... I think where I'm getting confused is determining something a bit more fundamental:  What is the actual topic.  I keep making that mistake when I look at the sentence structure.  I'm so concerned about where to put wa, ga, and o that I'm breaking apart the sentences and picking the incorrect words as the topic.  For example, I had assumed the word "Japanese" was the topic and not the word "you" - I think I'm getting too literal and not seeing the forest for the trees.

When I look at "you" as the topic, it makes sense to me.