Communities of the Light Ones and the Dark Ones can be interpreted as a political allegory. Here is a dialog from the first book "Night Watch", Story 2, Chapter 2:
Anton: «A Dark Magician can heal; a Light Magician can kill,» I said. «That's the truth. Do you know what the difference is between Light and Darkness?»
Svetlana: «No, I don't. For some reason, they don't teach us that. I expect it's hard to formulate clearly?»
Anton: «Not at all. If you always put yourself and your own interests first, then your path leads through the Darkness. If you think about others, it leads toward the Light.»
Contrast between interests of the society and one's personal interests is a typical comparison of socialism and capitalism. I don't see an analogy for the Watches in the real life though.
In the descriptions of major conflicts of the 20th century the Light Ones participate on the communist/socialist side:
Anton: A passionate heart, clean hands, a cool head… Surely it was no accident that during the Revolution and the Civil War, almost all the Light Ones had attached themselves to the Cheka? And most of those who didn't had died, at the hands of the Dark Ones, or even more often at the hands of those they were defending. (Night Watch, Story 3, Chapter 3)
Anton: «A new revolution?»
Olga: «We didn't want the last one. It was all supposed to happen almost completely without bloodshed. You understand: We can win only through ordinary people. When they become enlightened, when their spirit is uplifted. Communism was a wonderfully well-calculated system, and it's all my fault that it wasn't realized.» (Night Watch, Story 3, Chapter 3)
2. World War 2
Olga: «Of course. You don't get sudden eruptions of hatred like that during times of peace. That son of a bitch Adolf… he had plenty of admirers, but he would have been incinerated in the very first year of the war. And the whole of Germany with him. The situation with Stalin was a different case, adoration on a monstrous scale like that is a powerful shield. Anton, I'm a simple Russian woman…«—the smile that flitted across Olga's face showed what she really felt about the word «simple»—«and I spent all the last war shielding the enemies of my own country against curses. (Night Watch, Story 1, Chapter 3)
So while Olga was protecting Hitler during WW2, she was doing it on an order against her own loyalties.
3. Cold War The watches were not involved.
The puny conflict between the USA and the Islamic world was nothing in comparison… Even the old Cold War between the USA and the USSR that was now a part of history hadn't come close to the war of the Watches. They were just childish games for foolish human beings. (Day Watch, Story 3, Chapter 1)
On a few occasions the Light Ones are blamed for the failures of communism, and support of nazism. Such accusations are made by the Dark Ones, but are not admitted by the Light Ones. Such statements are either left without a response, or are rebuked. When the Light Ones respond they shift the blame for the tragedies onto the Dark Ones.
Zabulon: «There's no point in beating about the bush. In the last hundred years the forces of Light have launched three global experiments. The revolution in Russia. The Second World War. And now this new project. Following the same scenario.»
Anton: «I don't know what you're talking about,» I said. I suddenly had this desperate, aching feeling in my chest. (Night Watch, Story 3, Chapter 5)
Edgar: "At the time the Night Watch and the Day Watch were fighting for the right to conduct a social experiment," Edgar told me. "Communism, as you know, was invented by the Light Ones…"
Anton: "And subverted by the Dark Ones," I couldn't resist remarking. (Twilight Watch, Story 2, Chapter 4)
Semyon: «Very neat,» said Semyon. «I remember during the Civil War I was sent to capture a were-tiger. The bastard worked in the Cheka, and pretty high up too…« (Night Watch, Story 2, Chapter 1)
UPDATE 2 Arina's Story
Soviet Union is presented as a joint experiment by Night Watch, Day Watch and Inquisition. The Light Ones were hoping to build a better society. The Dark Ones were hoping that it would create hostility towards Soviets. Dark witch Arina was ordered to remoralize large number of people in USSR (e.g. make them believe in communism).
Svetlana: "What are you accused of?"
Arina: "On the instructions of an unidentified Light One I modified the recipe for a potion and so ruined a joint experiment by the Inquisition, the Night Watch, and the Day Watch."
Arina: "The experiment was supported by all the Others," Arina said. "The Dark Ones believed that the appearance of thousands of convinced communists in the leadership of the country-the bread plant's output mostly went to the Kremlin and the People's Commissariats-wouldn't improve anything. On the contrary, it would only provoke hostility to the Soviets in the rest of the world. But the Light Ones believed that after a hard, but victorious war against Germany-the likelihood of that was already clear to the clairvoyants by then-the Soviet Union could become a genuinely attractive society. There was a secret report… basically, people would have built communism by 1980…"
< snip >
Arina: "... but they were supposed to have built a city on the moon in the '70s. And fly to Mars, and something else. The whole of Europe would have been communist. And not under constraint either. And by now here on earth we'd have had a huge Soviet Union, a huge United States… I think Britain, Canada, and Australia were part of it… And there was China left on its own."
Anton: "So the Light Ones miscalculated?" I asked.
Arina: "No." Arina shook her head. "They didn't miscalculate. Of course, the blood would have flowed in rivers. But what came at the end of it all wouldn't have been too bad. Far better than all the regimes we have now… The Light Ones overlooked something else. If things had gone that way, then around about now people would have learned that the Others existed." (Twilight Watch, Story 2, Chapter 7)