16:36 Aug 3, 2011 

It might be useful qualify "advances" and "trench" to make the paragraph clearer. I suppose that you've already discussed those things earlier, but, in essence, what were they? Innovations? Tactics? I would start with "Roca's modern rifles/rifle assaults proved to be more effective", but after that, as an editor, I would want more background on the trenches and the advances.

16:32 Aug 3, 2011 

Here i copy the whole paragraph:
Before he was able to make his project come true, Alsina died and was replaced by young General Julio A. Roca. Roca would apply an annihilation plan against native communities through a systematic offensive war. Roca’s modern rifles were more effective than Rosas’ advances had been forty years before and Alsina’s more recent trench. In 1879, Roca headed an expedition to the desert and pushed the native people beyond the Negro river, to finally chase them up to Patagonia in order to crush their offensive power

16:25 Aug 3, 2011 

In order to give you a sound answer more information would be helpful.