Remember that Karl von Clausewitz lived and served as a
soldier(a general by the time he died) and military reformer for
Prussia during the Napoleanic era and post-Napoleanic era,where war
was still a viable tool of politics and policy(the original German
word used by him was politik,which means both in English),partly
due to the fact that firepower was still quite limited for
wholesome destruction. In one delves into history it is repleted
with wars and battles,in fact only until recently that the more
advanced states ceased and limit itself to utilize wars and battles
further one's interest or ideology.
Karl von Clausewitz's treatise which you derive your quotation
from,"On War",gained its popularity when Prussian Marshal Helmuth
von Moltke,who the Elder engineered the Prussian victory over
France in Franco-Prussian War(thus resulting in the unification of
modern nation-state of Germany under Prussia),attributed his
victory to the book(though it is noteworthy that von Moltke studied
in the military academy when von Clausewtiz was the principal,but
never taught by him.).
"On War" is pretty much a complex book to comprehend,due to the
fact that Karl von Clausewitz was a rather over-elaborative
writer(to the point of being messy and confusing),mistranslation
from German to other languages(esp. English) and that he
experienced the changing nature of warfare from the Napoleanic
era,one of unlimited warfare to propagate liberty,equality and
fraternity(at least on the surface),and post-Napoleanic era,one of
limited warfare in which states struggled to seize territories or
to obtain concessions or favourable treatise from other states. The
last factor compelled von Clausewitz to amend his treatise which he
had wrotes two decades ago,but he died of cholera before he could
complete it.
What it is miscomprehended(esp by "the Bear" and threadstarter) is
that Karl von Clausewitz is not advocating war in the quotation,but
delineating the nature of war itself,that "war is a continuation of
policy by other means"(in fact,by your logic,if i were to say that
flu virus causes influenza do you mean that i am encouraging you to
catch a flu? 
). As I mentioned
previously,"On War" is a humongous military treatise to
comprehend,words easily taken out of context(thus military
historian Micheal Howard commented that it is the most frequently
quoted but the least read book,much less understood by the mass)
overmore originally in German,thus a good translation with
annotation is very much important to understand it. Where English
translation is concerned the one Micheal Howard and Perot is
unarguable the best,even international affair researchers and
sociologist cite from it.
