That certainly was not the view Admiral Fisher and many others in the British Navy given the sinking of five allied war ships in March of 1915 in an attempt to "force the straits". Floating minefields, Turkish artillery along a strait 65 km in length and at the narrows 1,600 meters in width made the proposition of pushing through the Dardanelles daunting. In the book, Lawrence in Arabia, there is a convincing discussion that for Lawrence and other British officers on the ground in the area there was a significantly better landing site in Syria than Gallipoli. Playing "what if" with history and definitively saying there is a correct line of action in a difficult and rapidly changing combat situation is the province for those who don't have to execute that line of action.

The best historian on this is Robin Prior, whose book Gallipoli: The End of the Myth ...erm, ends the myth. To paraphrase him: it was essentially impossible to win in the Dardanelles, because those planning had no real conception of what victory would look like. It was begun as a purely naval operation, a means of bringing the might of the Royal Navy into the war in a meaningful way. Except, the problems here were manifest: using Dreadnought-type ships was a huge risk because, if they were sunk, it would fundamentally shift the balance of naval power. Using pre-Dreadnoughts was a better idea because of this, except they didn't have the armament to knock out the Ottoman forts along the Straits. Hence, the landing of troops. But, let us presume that the troops succeeded, that the forts were knocked out, and that the navy successfully proceeds up the Dardanelles to the Bosphorus. It was presumed that two things would happen:

• If the navy turned its guns on Constantinople, the Ottoman Empire would surrender (especially important since no one in Whitehall could countenance that the Royal Navy might bombard Constantinople!); and

• This move would embolden the small Balkan powers to unite in a common cause and strike towards Germany.

Both of these ideas were faulty. Our best evidence suggests that the Ottomans had planned for the eventuality that Constantinople might be besieged, and had already made plans to continue the fight by evacuating to Anatolia. But in any event, the second premise - that the Balkan powers would unite - was ridiculous. Most of these states had only recently been at each other's throats. They were unlikely to band together. Moreover, what could they have accomplished? The War Cabinet seems to have thought that they could have done miracles - somehow crossing through Austria-Hungary, then over the Alps, and thereafter into Germany. In truth, most of these small states (Rumania, Bulgaria, Serbia, Greece, and so on) had such small, decrepit armies that they could not have done much of anything to support the Allied cause.

So, even if everything had gone like clockwork, the end result would not have been some decisive victory that would have shortened the war by any measure. As Prior argues, the main enemy was Germany. The war was to be won or lost by fighting Germany. The Allied powers could not win the war by defeating the Ottoman Empire, but they could certainly lose the war by not defeating Germany.

Well it's far from obvious what would have happened if the March 18th naval attack had been pressed on or after March 19th. The Ottomans were very concerned that a naval attack on their capital would cause the disintegration of the Empire. However, it's really impossible to say whether these concerns were valid. Different British Commanders on the spot had very different opinions of the likely outcomes of further fighting.

I think you're right to be cautious about William Manchester though, he is more journalist than historian at times, and you never quite know when he'll cherry pick the juicier stories or put something in an odd context. Manchester is not always bad and The Last Lion hasn't been caught out in too many errors, but he's not entirely reliable either, especially in a few of his other works.