A "decisive victory" refers to a clear and definite win in a competition or conflict, where one side achieves a conclusive and undeniable success over the other. Full definition
In a world facing almost daily new evidence of global warming and its consequences, there is a need for a quick decisive victory in the effort to cut carbon emissions and stabilize climate. (earthpolicy.org)
He has been the favourite for so long that everything but decisive victories ends up being framed as a bit of a disappointment. (blog.politics.ox.ac.uk)
That might not seem like all that much, but it was 68 percent of the vote, which is a pretty decisive victory. (nystateofpolitics.com)