Complex sentence: The American view was that defending convoys did little to reduce or contain U-boat numbers, while the British were constrained by having to fight the battle of the Atlantic alone for the early part of the war with very limited resources.
Split into simpler sentences: The American view was this tactic did little to reduce or contain U-boat numbers. The British view was influenced by the fact they had had to fight the battle of the Atlantic alone for much of the war, with very limited resources.