Halleck is a "conundrum" if you have twisted the early war historical record into a storybook shape and dipped it in honey.
If you've done that, then you have the problem - conundrum - of a decisive and active combat commander transformed into a shirker, starting with his promotion to general-in-chief.
But if you follow Halleck's career carefully, it ends as it begins with hardly a bump in between. There is as much shirking in St. Louis as in Washington.
Remember who and what Halleck was.
He began as a political tool used by Lincoln against Fremont; once Fremont was out of Missouri, Halleck did excellent service blackening his reputation for honesty and competence and dramatizing his own herculean efforts in cleaning up after the Pathfinder.
During McClellan's tour as general-in-chief, Halleck represented the McClellan alternative. He was the g-in-c Scott wanted; the military genius who would do political dirty work; the steward of Western victories; and the man who would secretly meet with Stanton and Lincoln behind McClellan's back to give them advice.
After selling the idea of a consolidated Western command (Mac had nixed it), Welles says Halleck was brought East through the intrigues and patronage of John Pope working with cabinet powerhouses Chase and Stanton.
Halleck, as new g-in-c, well understood himself to be between a rock and a hard place, between a subordinate patron (Pope) who needed a lot of help and a politically formidable former boss (McClellan) who had wide newspaper backing and who talked a lot of sense. If he dithered in Missouri, he would certainly dither in these circumstances.
But all this ignores the matter of Lincoln; he had tried several ways of working and found what he wanted to do. Halleck's activity embraced Lincoln's preferred way of operating; it was not a letdown, it was a fulfillment, which is why HH lasted so long.
People fail to observe how Lincoln managed and how he arrived at difficult decisions. Lincoln spoke to people who in his mind represented entire constituencies - he leveraged a belief in political personification to get work done. This was a tool in his "master politician" kit.
In that highly personalized representational system, Halleck represented The Army. The Whole Army. All the time. And that's all he needed to do or be in order to be invaluable to Lincoln. Lincoln could circulate through offices all day long collecting opinions on any and all matters, and there would always be a Halleck within reach to supply answers for an entire, vast constituency: "the Army's opinion."
You won't see this in military histories, but one often encounters Halleck attending to non-military matters - here, he's with Seward discussing something, there he's with Chase - suddenly Lincoln finds him and they all together take up the matter of Navy construction, or taxation, or the appointment of a civilian somewhere. Halleck is not hard at work doing staff work, as would some "first-rate clerk," he's wandering around smoking cigars and scratching his elbows - making himself fully available to "browsing presidents" (in Mac's terms) and to kibbitzing cabinet officers.
If historians had studied the best part of the McClellan-Lincoln relationship, if anyone besides myself ever bothered to count their daily visits and messengered interactions between 8/61 and 3/62, they would understand the extremely painful void in Lincoln's life that Halleck filled when he arrived to take up the job of g-in-c.
The functions McClellan assumed from Cameron's War Department had returned to the Stanton's War Department with interest. What was left was the part of the job Lincoln valued the most.
Halleck was a tremendous success. And that had nothing to do with staffwork.