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<p>Case-by-case basis. Very, very few have been allowed to do this – and all in different ways. The ones that come to mind are Staubach, McConkey(?), McCallum, Robinson, and now apparently Eckel. About 1 per decade across all sports.</p>
<p>I believe that Staubach did his (then) 4 years on active duty before going to the NFL. Spent his 30 days of leave each year at Cowboys’ camp. As I recall, McCallum and Robinson each did 2 years of active-duty service before going to the NFL and NBA, respectively.</p>