Very true Stateless_englishmen, I didn't want to delve into massive lists just a general overview, it's just Fort Tourgis is preaty imposing;
http://www.cipostcard.co.nz/p205s.jpg
Thankyou
Taken on board.
What in general people are eluding too is how many troops would the British garrison the islands with, in comparison to the prepared German invasion forces?
We know in real life the Germans stationed about 26,800 men across all the islands by 1943 (13,000 Guernsey, 8,850 Jersey and 2,850 on Alderney) (although there were also an additional 16,000 Organisation Todt workers on the islands).
Initially in 1940 there was only about 1,700 on each island (although there was also about 1,100 Luftwaffe forces there). By 1942, it was 36,960.
Now this has a lot to do with Hitlers madness that the islands be made impregnable against attack. But assuming the reverse from Churchill that not one step could the Germans be allowed on British soil, we could expect something in about the same orders of magnitude.
Recall that pre-war planning had planned rationing for over 97,000 people to be present on the islands. Not only that, the islands were already at peak agricultural production levels.
Therefore in terms of supply, without a civilian population to eat up supplies the islands would become very resistant to being sieged out.
Indeed if we consider real life. The German garrisons on the island only surrendered the day after VE day May 1945, and remember D-day was way back in June 1944. The Germans were able to remain besieged for over a year! and they were still actively at their posts right up until that time*.
Now while it did get tight towards the end of this period and red cross parcells were sent the estimated calorie count per day was still fairly high at 1,137 (compare that to other sieges), the islands were totally out of supply due to the allied advance, and you did have large numbers of German troops and civilians to be fed. Reduce either of these, and it is likely the garrisons could hold out even longer.
It is important to realise that the Germans Occupying the islands only surrended because of the fall of the 3rd Reich at the time. It is unknown how long the siege may have had to continue untill the garrison would have had to surrender.
Incidentally all the islands have fresh water sources. Water doesn't need to be imported, food was also fairly self sufficent, but medical supplies or manufactured goods were the main imports that the Germans needed to bring in.
The end result, is if we assume that a British garrison is very simmilar to a German garrison, it is likely the British garrison could hold out just as long and so besieging the islands would unlikely have been a winning stratergy for the Germans.
The British would probally recognise if the German Luftwaffe was targeting the merchant shipping about the islands, and then take steps to give the merchant ships the required escorts to deter those kind of attacks.
However realise that the English channel at this point would be very 'British waters' and hostile to the German Luftwaffe because the Islands would present a forward base, even if there was no radar instilation on the islands, the fact that all three have airfields means that if one island spied incoming German planes, they can scramble from all three islands. Plus reinforcements are only ~20mins away from Cornwall.
All in all, the islands would present a difficult location to maintain air supiriority over for the Germans.
U-boats, assuming Alderney is turned into a ASW base, would be folly because the ships at Alderney would actively be searching for U-boats in teh Channel. Therefore making it even harder to cut of supply.
This doesn't mean to suggest that the Germans wouldn't try to beseige the islands, but it would go to suggest that the tactic would ultermatly be unsuccessful and not cost the Allies a huge amount in the long run.
Having said that Hitler was very much for placing 'super guns' on the islands to cover the French coast in that region as part of the Atlatic wall. It might be fairly likely that the Germans would bring in heavy artillery into that region of France and try and bombard the islands (it would be unfessible to to bring big guns onto the islands, hence why the Army scrapped Hitlers ideas quietly).
Since the islands themselves were never bombarded during the war, we cannot say what the effect of that would be like.
However we could draw some parrellels with operations in the Pacific where the big guns of the US Pacific fleet were generally ineffective at reducing the capacity for the defenders to fight.
Of the islands Guensey and Jersey are fairly 'hilly' with areas of steep cliffs, sand dunes and an elevated plataeu from the beaches. Then you have effectively the same equvilent of the french bocage terrain through the areas of fields.
So that is about the best comparisons that can be drawn.
It is likely in my opinion that niether side would have made any moves, unless Hitler had ordered a suicidal attack on the islands (since one could not be taken without the others really) and that would have meant a stalemate.
If there had been an invasion it would have been tremendously costly for the invaders.
Something touched on by I'll highlight now is the tides. The Channel Islands have the 3rd highest tidal range in the world at 12-13m you might say so what? But that means that at low tide, you've got about 1km on the wide flat beachs from shore to sea, and in some places it goes out 4km! Most of these beaches also have associated towers, and the Germans saw very quickly the worth in constructing more machingun pillboxes to cover these wide expanses.
At high tide, the water came up right to what was then, the Dunes at St. Ouens and along St. Lawerence and Gorey (this is Jersey, I'm not so certian on Guensey here) while again the Germans realised was perfect to mine, the terrain itself is sandy and difficult to run in, and nearly always overlooked by cliffs of the central plateau (again they put pillboxes up on the cliffs).
So what about landing mid tide?
Well firstly your not going to land on the ebb, otherwise reinforcements are placed futher and futher from the fighing. So your going to land on the incoming tide.
Taking into account the rule of twelfths you've got a major problem with that and a high tidal range, because if your soliders don't press forward, then they are in serious danger of finding themselves 3m underwater, and struggling in the surf!
Indeed the British commandos found that one out for themselves on raids on the islands, but any local could have told them that right away.
If the Germans worked that above out they would have seen the folly in trying to take the islands, much like what the British realised about trying to retake them. Hence with good intelligance you don't get any invasion, with poor, you'd end up with a hell of a lot of dead before you even got off the beaches. Then you still have to fight up the headlands, and if you know about Pointe du Hoc during D-day I wouldn't envy being a German trying to scale our islands cliffs!
Like has been mentioned an air assult would be the most sensible course of action given all of this, but doing so against islands that have their own RAF wings, or once you land are completely on a do or die mission, and if you know about the invasion of Crete that didn't go to well either.
On the islands you wouldn't be able to land gliders effectively either due to small fields and hedgerows. You still then left having to take many of the fortifed towers and forts on the island, some of which are out to sea themselves so either wait for low tide and get machinguned, or get in a local boat and have the same issue. Kein problem!
* That kind of makes what you say Peg Leg Pom inaccurate, historically the Germans
did hold the islands without the peninsular. It is true that attacking the islands would have been a waste of lives and this is why historically the British didn't attack them.
It is likely without input from Hitler, the Germans would see British defended islands in the same way, and not bother attacking them.