That seems to be the working number for the residual force. That is the strength of two full divisions - around 13 or 14 brigades worth. That's a big twinkie.

Some key questions:

How will this residual mission be framed? What are the rules of engagement and what is the level of authority to use heavier firepower to assist the Iraqi army?

If that is the residual force size, will the anti-war crowd accept the trickle of casualties, occasional combat operations, and the budgetary costs this will require - for years?

Finally, if the day comes, will we use air power to crush AQI/Sunni insurgent/Shiite militia/Quds Force armored columns heading towards Baghdad? Or will the pleas for assistance land on deaf ears in the WH - the way they did in 1973.

Aside - Hope all is OK. Really enjoying your book. The backstory on ADM Fallon is fascinating! So little of that came out in the media as it unfolded. Seemed at the time they were looking for another Shinsheki-like hero.

Your book is great as far as it goes but it appears you had a publishers deadline and it just ends. Where is SOFA talked about? What is next? You question Obama but he gave a plan:
Please read the Obama speech:Remarks of Senator Barack Obama: Turning the Page in Iraq Clinton, IA | September 12, 2007.

He spoke of the residual force combined with diplomacy. This thing is not going to end with a military victory. President Obama outlined the strategy in that speech why didn't you address it in the book or now? We have to bring in the UN, Iran,Syria,Turkey and Saudi Arabia.

PS
Your "CS" comments about combat vs. support forces is something I didn't expect from you.

Equating a raw number like '40K in 2011' with with a brigade count of 13 seems wonky. We have 15 brigades in Iraq today, but that's only 50,000 out of 140,000 deployed. That gives us a brigade to total force ratio of about 1:3

As you start bottoming the force, all the manpower tends to go for security and force protection, leaving little for mission execution. At the top end, the 'surge' added 5 brigades, roughly 15K out of a 30,000 swell in the deployment. The 1:2 surge ratio was an overload effort, using existing MNFI infrastructure built up over 5 years, and the urban deployment reduced the relative demand for armored transport. Some guys were walking to work.

In mid 2006, the well informed Stephen Biddle was talking about 100,000 in Iraq as perhaps sustainable, and building a long war strategy around that kind of number. Since we then had 15K in the Kush, and the force structure has expanded a bit since then, I'll guess that 120-130K is a max sustainable effort for our surge attenuated ground force, for both wars. (We'll leave the 5K we have in Columbia, and the handful of deployed MEU's out of this.)

So if McKiernan gets and supplies his 55K (plus NATO) for Afghanistan, and Iraq stays sort of quiet with 50,000 in training/overwatch, that finally leaves a coupla brigades on deck as reserve, and some ability to rebuild our NCO base.

Not exactly the reset we hoped for, but that would be a major shift towards sustainability. As opposed to the 200K+ 'full throttle and losing altitude' condition that we face at the end of the Team Bush years.