'Asymmetric warfare' or more aptly, 'asymmetric conflict' involves a confrontation between two sets of agents in which one set possesses vastly greater resources. In more recent time, the notion of 'asymmetric conflicts' involved the less endowed agents winning against more endowed ones. And the degree of asymmetries has grown significantly over time:
In Vietnam War, vastly outgunned Vietnamese forces literally defeated vastly over-equipped French and U.S. military machines;
In the Cold War confrontation, significantly less resourced Warsaw Pact managed to sustain relative long-term parity with much more resourced Western counterparts (including Nato);
In post-USSR years, vastly under-resourced Russia, compared to vastly over-resourced U.S. has been able to achieve quite a few 'wins' in geopolitical arena;
Isis - with barely any resources, has managed to achieve huge gains against a range of much better equipped counterparties;
In Afghanistan, Taliban - with military expenditure of just a few million per annum, is successfully holding the line against both the Afghan state and its backers; and of course,
The 'rust-bucket' North Korea has just outplayed the U.S. in its race for nukes as a deterrent.
In summary, thus: spending does not secure reduction of risks in the age of asymmetric conflicts.
Now, consider the two key sources of 'existential' threats to the U.S. geopolitical positioning in the world: Russia and China. Illustrating asymmetric conflict:
And despite this obvious lack of connection between volume of spend and outruns in terms of geopolitical achievements, the prevalent consensus in Washington remains the same: more funds for Pentagon is the only way to assure preservation of the U.S. geopolitical positioning.