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human
[Doctor Who] How was the Time War fought?

Recently started watching the new Doctor Who series and saw mentions of a Time War being the reason the Time Lords no longer exist.

Can someone explain the Time War? How does an entire race of time travelers get exterminated (heh) in the distant past? Shouldn't there be refugees scattered all over the timeline hiding from the conquerors?
gpt
It's an effect of two different phenomena.

First there is the tendency for all time wars to travel further into the past. Even for time traveling societies, effects tend to be later than their causes, due to the default method of time travel, one second per second, forward only. You have to go out of your way to make an exception. This means that the conflicts tend to have their triggers sometime relatively late in the relations between the belligerents. Since the people initiating the time war are from this later era, their incentive is to defeat the enemy as recently as possible to maintain their existence as close to as it was when the war began, with the exception of the victory over their adversaries of course.

However if one side is victorious, say the Daleks are at some time in the twenty-second century, this can be undone by defeating them earlier on in their history, such as the twentieth century. Likewise the Daleks can undermine the Time Lord victory by defeating them in the nineteenth century. And so on back through the ages.

This brings us to the second part where things get all wibbly wobbly. As a general rule for avoiding potentially devastating paradoxes, a time traveler avoids crossing his own timeline, that is to say to avoid disturbing his own causal chain. However as move and counter move are made in a time war, these causal chains get more and more entangled, so that fewer and few possible safe moves are available, as altering another time traveler's past, or future, would be altering your own. This is the non-linier counterpart to the drift towards ever earlier battlefields.

By this time people are not as concerned about such things as paradoxes, but time itself still is. With a concerted effort paradoxes can be maintained, but as cross-time conflict becomes more complicated, the ability to change anything becomes more and more difficult. 

A lesser version of this is responsible for so called "fixed points" in history. These are events that are so important to the interwoven histories of time traveling societies, that they are essentially locked in place by the interconnected causal chains. 

It is to these fixed points, these common causes that the war is drawn, as no other field of battle would be effective, and increasingly, would not be possible. In the case of the Last Time War, this focused on the golden age of the Time Lords on Gallifrey. 

As the convoluted causal chains more and more tightly intertwined around this period, it became impossible to change anything, as any action one might take, was already part of the causal past that had resulted in the inescapable, self-contained, and self-consistent constellation of nows centered on the golden age of Gallifrey. 

The common term for this phenomenon is "time lock".