How does one issue a call for papers on military history?
Some of your expectations fall within the norms and some may not. There may be some variation by field.

Having never organized a conference, but having attended and presented as several, these seem to be the norms I have encountered:


  Where would one publish a "call for papers" on specific areas of military history? 


Anywhere and everywhere. Send emails (or snailmail) to the history departments at nearby colleges and universities. Pass the word along to other organizations with similar interests. Are you close to a military academy? Is there a military museum nearby? Those are other good places to find speakers.Make sure that all correspondence includes the soft deadline (i.e. about a week before your real cutoff) for submission, and the process by which an individual signs up to speak. Also, you need to make clear the type of talk you are expecting based on the audience. Should this be a talk for experts, for enthusiasts, or for the general public? These announcements also serve as advertising for your event.


  What would be expected to be provided for presenters who are accepted? 


Generally, at large conferences with hundreds of talks, the travel costs are paid by the speakers. For a small conference, you may want to offer to reimburse some costs on a first-come-first-served basis, however, very few folks should be put out by lack of financial support. I would save honoraria for their true purpose: providing an award for someone and/or attracting a really big name.


  Would sample presentation videos be excessive? 


Yes. The situation you describe is a known danger, but it is not normal to ask for videos. Your better bet is to send people to attend other talks by your speakers and evaluate them unofficially, but you should still let them speak. If you secure a big name in the field, you will attract an audience, even if said person is a horrible speaker. If you are attracting academic speakers and you have a nearby university, asks folks in the relevant departments if they know of good speakers and try to target those individuals.

You might get away with suggesting that individuals who wish to be considered for the keynote spot send in a short clip of them presenting elsewhere, but that would still be pushing it.


  How long a presentation would be acceptable from the speaker's perspective? 


Depending on the number of speakers you hope to attract and the length of the conference, block off 30 minute or 60 minute blocks. 15 minutes is a little short, and you should only exceed one hour for keynotes/plenaries. Most academics would interpret a 30 minute block of time as 20 minutes of presentation and 10 minutes of questions and answers, which sounds like what you want.