hi thucydides, i havent read (thucycides) before, but i'll take a guess at the basics of this sentence, since it doesn't seem too tricky, and others can fix it up and give fuller advice.
the perseus translation is (including the part of the sentence leading up to it):
(This mode of attack the Syracusans had with good reason adopted in preference to fighting at close quarters,) as to risk a struggle with desperate men was now more for the advantage of the Athenians than for their own; besides, their success had now become so certain that they began to spare themselves a little in order not to be cut off in the moment of victory, thinking too that, as it was, they would be able in this way to subdue and capture the enemy.
[/face] is the article + inf., "a struggle".
[face=SPIonic]pro\j a)nqrw&pouj a)ponenohme/nouj
[/face], "with desperate men"
[face=SPIonic]ou) pro\j e0kei/nwn ma=llon h)=n e)/ti
[/face], "(was) not more to (the advantage of) them"
[face=SPIonic]h4 pro\j tw~n 0Aqhnai/wn,
[/face], "than to the Athenians,"
[face=SPIonic]kai\ a#ma feidw& te/ tij e0gi/gneto
[/face], "and at the same time "some sparing began" (or something like this)",
[face=SPIonic]e0p' eu)pragi/a| h1dh safei=
[/face], "(since success already certain)"
[face=SPIonic]mh\ proanalwqh=nai/ tw|,
[/face], "not to be cut off,"
[face=SPIonic]kai\ e0no/mizon kai\
[/face], "moreover they thought that"
[face=SPIonic]w$j tau/th| th=| i0de/a|
[/face], "(thus in this way"
[/face], "to capture them" (the inf. here goes with the "they thought" bit as the indirect discourse).
hope this helps