This could have gone into either Cafe or GQ:
How well is a military such as the United States’ (or IDF or some NATO ally) capable of coping if a sneak enemy attack happened to kill all generals/admirals/colonels/etc. at a single location?
Would the military just promptly promote the low-ranking brass to higher positions and carry on?
Well Stalin killed off most of Russia’s top brass just before WWII. A few years later Hitler broke the treaty with Russia and attacked. All and all the Russians did pretty well.
Also all the US upper brass are seldom at the same location so killing them off at once would be difficult. There’s enough 4-star generals and Admirals that we wouldn’t need to promote low-ranking officers right away.
Sure. There are tons of officers who are just slightly less capable than the ‘top brass’ on an OER who got passed over for the promotion to General, etc. They would do fine filling the shoes of their deceased superiors.
There’s also retired officers and generals. Retired officers can be called back to active duty as well.
Other than eliminating those officers with detailed knowledge of current operations, it might actually improve functioning in some instances. 
Not at first, though. It took them a couple years to get their act together.
Yes, but only some of those guys know what’s happening in the specific theater. Wiping out the theater HQ (or equivalent in non-US countries) would be a big blow to the performance of just about any military.
Norman Schwarzkop’s biography, It Doesn’t Take a Hero, details his training and progression up the ranks.
He tries to show how various commanding officers influenced his career. What he learned at his various postings.
A lot goes into creating a General. Norm almost quit several times. Ultimately the right men were in place at the right time. Desert Storm was lead by Generals that had served in Vietnam early in their careers. They can were determined not to make the same mistakes.
It would be challenging to replace Generals. But there is a large pool of similarly trained men that can step up and fill the role.
What you would see is a lot of high ranking retirees getting recalled to active duty.
Plus, at least in the American military, NCO’s are just as vital as generals, can and do command troop movements, dictate battle tactics, etc.
[tangential hijack]
Just wondering why OP thought this question might belong in Cafe? 
[/tangential hijack]
Can’t think of many instances in US History where a large number of senior commanders got killed. Well, there was the Confederacy at Gettysburg, but they were rather adamant wrt “we are not the US”. It did hurt them.
The UK suffered quite a few General officer deaths at the Battle of Loos and in the preceding operations. Several Brigadiers and Major Generals. Which meant that there was a massive shortage of good and experienced men in high command during the 1916 battles.
And while the brass are the ones In Charge of the big Plan, the person who’s actually got most of the files in his computer might be the lowly first lieutenant who had to create the Power Points of Doom. Step one of solving the mess: realizing that information is there; step two, get computer and lieutenant to another qualified piece of brass (or other shiny metal of slightly lower quality).
Since it was somewhat fictional.
This isn’t entirely a joke, IMO.
The leadership qualities that lead to promotion in a peacetime military are not the same ones needed by a military at war. In peacetime, it’s mostly don’t make waves, don’t take risk, don’t do anything that would cause attention/embarrassment for your superiors. But in war, those priorities are nearly reversed.
I recall reading that after Pearl Harbor, the US Navy, especially the submarine forces, had difficulty with this. Many sub commanders were old, cautious, and risk-adverse, and the submarine force wasn’t very effective until they were replaced by younger men, or learned new priorities for their commands.
Is there a rough estimate for the number of retired officers vs active duty?
Also if an officer retired 20 years ago, isn’t their knowledge of military technology going to be totally out of date?
This issue came up in a book I was reading about navy SEALs fighting the war on terror. Who knows if they were just exaggerating, but according to them their officers were very risk adverse and not willing to actually go in country. A foreign special forces unit they knew said they were like ‘lions being led by dogs’. I have no idea if that is a major issue in wartime though.