Afghanistan Enters WWI on the Side of the Central Powers

In either 1915 or 1916--indeed, you decide.

That's certainly not going to prevent revolution from occurring in Russia in a couple of years, though.

Yes, I certainly know what you meant here.

The Ottomans and Germans had been trying to woo Afghanistan once the war broke out. The Ottomans wanted to use the Afghans as a way to expand their holding within Persia, Central Asia, and the Russian Caucasus. The German diplomat sent to Afghanistan (Baron Max von Oppenheim) Said that the population was not one to be underestimated as they were a powerful and prideful people. They figured that if the Afghans managed to penetrate the Indus Valley that India itself would rise up in rebellion against the British rule.

The Russians were facing issues along it's border with Afghanistan already, mainly with the Kazakhs and a few other people's groups that rose up against being forced to join the army, their main population base was within the central Asian mountains and gave a large (if not weak) barrier to Russian operations against the nation.

To answer the question, you would be looking at roughly 140,000 men (50,000 being the regular army with a supplement of 90.000 tribesmen that would be mainly armed with spears, swords, old firearms etc.) They would be attacking into the Pashtun region, an area that was high in terms of national identity and (albiet unlikely) willing to rise up against the British. The Pashtun people created seven regiments (70,000 men) during the war and five were sent to the western front. One was sent to Mesopotamia and the other to Egypt. So let's say that another 30,000 actually decide to join the Afghan forces (the population of the area is rather small even today, I don't have the exact numbers though) so that's 170,000 men attacking along a 100 mile front. They would have no hope of getting really any supplies from the Central powers as Persia was under partial occupation by the British, and they had an intensive spy network already in place. The English could muster ~120,000 men in India if need be, Portugal had three regiments within their territories in India and depending on the time are part of the Entente, and if worst comes to worst, the Japanese would be more then likely willing to send aid as after 1914, the Japanese had little to no fighting left to do.

  I don't think Britain would make it an open colony, especially not while a friendly Russia was an ally.  However its likely that the Afghans would be heavily defeated if they tried an actual invasion of India and that British influence would increase afterwards as a result.

I'm saying that will become a protectorate...

  Or probably, from Afghan history, several minds of their own.  However think technically Afghanistan was already a protectorate of Britain.