Because of the
Great Game we tend to see 19th West Asia as a backwater run by two European capitols. But that ignres the simple fact that the two most powerful nations on the Earth were focused on dominating this area.
Let Iran be a real power, including getting railroads when the Shahs first sought them (the 1870s from what I can tell) and it radically alters the planetary power balance. Iran's alliance either way breaks the balance. Let Iran be more thoroughly shattered and the territory of Iran as absorbed by Britain and Russia stripping them of any buffer in an area were both of them can get signifigant numbers of troops.
If that statement seems odd, remember the Volga, Russia's greatest river, flows into the
Caspian Sea. Even in the 18th and 19th century it would be easier for Moscow than London to get troops to Iran. Once conected to the road systems of the Iranian platue (which in period were better than Russia's) a Russian Army could march to India. Yes, the logistics would be murder, but it is possible, if unlikely.