Actually he does it in the text. And his argument is pretty good one too, especially for a conservative. The connection between progressivism/liberalism and imperialism is a point well taken, albeit it suffers from the half-full/half empty bottle dilemma. Yes, some of the progressive rhetoric was used to legitimate imperialism, but some it was actually against it. The same can be said about the conservative rhetoric.
Woods seems to forget that conservatism-liberalism and isolationism-expansionism represent two orthogonal or independent policy dimensions. That is to say, it is possible to be on one extreme of one dimension and at the same on the either extreme of the other one.
Another problem of Wood's argument is the causal claim that political ideology is the root cause of directions of foreign policy. I think that is a fallacy commonly espoused by political scientists. A more accurate view, imho, is that viewing the policy process as a garbage can (see the garbage can theory of the organizational behavior). That is to say, because of many conflicting interests and agendas of various interest groups involved, the actual course of foreign policy the one that is most aggressively pursed by special interest groups and at the same time offers the least resistance to all stakeholder groups, but then it is ex-post-facto justified and rationalized by ideological claims and references that are appealing to various interest groups.
Thus, a decision to take a military action overseas will be justified in terms of defending US superiority and sovereignty to the right wing audience, and in terms of humanitarian assistance or defending human rights to the left-wing crowd. But the bottom line is that neither of these concerns may have been the actual reasons behind the decision in question. That decision might have been taken for very narrowly defined special interest groups, such as cover-up of a scandal ("wag the dog"), profiteering Halliburton-style, or perhaps doing favors to foreign friends or exceptionally vocal ethnic minorities in the US (e.g. Cuban exiles).
Wojtek