Why hold'claim Osgiliath?

[William Cloud Hicklin]

Quempel, the Osgiliath bridge had been destroyed (by Boromir) the previous June. In March Sauron forced an amphibious crossing.

[Quempel]

That's why I said access across the river, not bridge. There are certain things that can be done to keep boats from reaching the shore. There are certain things that can be done to shore up the shore. There are certain things that can make it impossible to cross a river except at a very narrow crossing, thus making it 'easy pickins' for the defenders. Again there are tons of military mistakes in both the books and the movies. However, having the giant eagles of Manwe fly the ring into Mt. Doom doesn't make for a good story. One of my biggest is the whole ring issue, why couldn't Sauron just as easily forge a new ring to rule over the one ring? Plus the wizards had knowledge of black-powder, I am pretty sure black-powder bombs or guns might just make it easier to win the war. But Tolkien hated that technology so it wasn't used, he preferred the killing in LoTR to be done with knives, not guns.

[William Cloud Hicklin]

Qempel, I think we can assume that Gondor tried to beef up the landing-zone defenses as best she could (although Denethor wasted manpower and time on the Rammas)- and apparently they held for two days; but with an enemy of overwhelming numerical superiority (as well as the attested morale-effect of the Black Captain), not to mention the fatigue of unrelieved troops in continuous combat- they just weren't capable of holding. There is no such thing as an impregnable position given sufficient men (and sufficient disregard for casualties). The idea that a few hundred Men of Gondor could prevent indefinitely the Morgul-Host from effecting a crossing is a non-starter.

Rommel did his level best to make Normandy 'impregnable,' remember?

why couldn't Sauron just as easily forge a new ring to rule over the one ring?

Because it wasn't 'easy' the first time, in fact Sauron invested 'the better part' (i.e. most) of his native power in the One. He hadn't sufficient left to make an even mightier one. Besides, in Tolkienverse some works, like the Trees and the Silmarils, can't be repeated.

Plus the wizards had knowledge of black-powder, I am pretty sure black-powder bombs or guns might just make it easier to win the war.

"The wizards?" Say rather Saruman. If indeed the 'fire of Orthanc' at Helm's Deep in fact was gunpowder, which we don't know.

[Nerwen]

Originally Posted by William Cloud Hicklin

Because it wasn't 'easy' the first time, in fact Sauron invested 'the better part' (i.e. most) of his native power in the One.

This being why destroying the Ring did for him.

[Gordis]

What seems curious to me is that the last Bridge has been built not too long before the War of the Ring:

For that is the ruin of Osgiliath on either side of Anduin, which our enemies took and burned long ago. Yet we won it back in the days of the youth of Denethor: not to dwell in, but to hold as an outpost, and to rebuild the bridge for the passage of our arms. And then came the Fell Riders out of Minas Morgul.- Minas Tirith, LOTR

Denethor was born in 2930. Sauron returned to Mordor in 2942 and declared himself openly in 2951, when Denethor was 20. What a crazy idea it was to rebuild the bridge at this time! Makes me think of Turin and Nargothrond...
Cast the stones of your pride into the loud river...

But the other side made strange moves as well. What was the need for Sauron to attack the Bridges of Osgiliath a year before the war, in June 3018? In UT it is explained like this:

So it was that Sauron prepared two strokes – in which many saw the beginnings of the War of the Ring. They were made together. The Orcs assailed the realm of Thranduil, with orders to recapture Gollum; and the Lord of Morgul was sent forth openly to battle against Gondor. These things were done towards the end of June 3018. Thus Sauron tested the strength and preparedness of Denethor, and found them more than he had hoped. But that troubled him little, since be had used little force in the assault, and his chief purpose was that the coming forth of the Nazgûl should appear only as part of his policy of war against Gondor.

Well, the nazgul may have feared water and all, but I don't buy it that there was no other way to ship them over the Anduin than using the Bridges. After all, Khamul and his pal from Dol Guldur did use a boat and were none the worse for it...
And the disadvantages were enormous. Basically Sauron gave ample warning to the Gondorians hitting a year in advance exactly in the same place where he planned his main attack for the next year. He sacrificed the Bridges, which he otherwise could have used next year. I guess the WK had a hell of a time shipping Grond and other heavy equipment over the Anduin in March 3019.

[William Cloud Hicklin]

It was hardly giving up some secret to attack Osgiliath in June- Osgiliath was almost necessarily the main focus. Any campaign against MT from Morgul would necessarily rely on the Road and its causeway, which meant Osgiliath.

Moreover, Sauron's plan in June was to seize the Bridge, not to have it destroyed- which would indeed have been an immense prize, and which he would have gained but for Boromir and Faramir.

Tolkien himself admitted that the Nazgul/water business was 'hard to sustain'- but the basic argument, that the assault provided cover and disinformazia for the RW's hunt for the Ring, still holds up. Nor is there anything at all wrong with a reconnaisance in force to probe the enemy defenses.

[Beanamir of Gondor]

What mystifies me more than anything else about the bridge issue is that, in RotK (maybe it was only in the EE, I haven't seen the original in so long that I don't remember!) there's an image of trolls and Orcs pushing siege towers and Grond, among other things, across what is clearly a bridge, in what is clearly Osgiliath. Is it a bridge over the river, because it certainly looks like it to me! Next time I watch the movie I'll mark down the time, but it's about, eh, four or five scenes before the end of the first disc. I was mystified about this even before reading William CH, Quempel, and Gordis's posts (and having reread RotK just last week while crewing a show), because while watching the films I vaguely recalled that there was no bridge in Osgiliath... and now I'm just peeved, since I know that there definitely wasn't.

I'll have to agree with William CH, though... in the films, it came out like "a ludicrous suicide mission is the most convenient way for Insane Mean Daddy to kill off Misunderstood Younger Son." Particularly because the whole "Yes, I do wish that," scene was horrifically skewed from in the books, and Denethor really did wish his son was dead. Blahhh, don't get me started on that.