We must remember that to our mind, coming from the 21st century, the majority of the scenario that Mark unveils to his brother in the second chapter of this excellent novel is going to sound very dated. Clearly, we need to remember that this novel was written at a very specific point of history, one that we have left long ago, and that now weapons technology has advanced to a massive extent. In particular, we might question the way in which Mark paints a picture of the Russian strategy of firing missiles so that they all arrive at their targets at the same moment:
Time-on-target. You don't fire everything at the same instant. You shoot it so it all arrives on target at the same instant.
Nowadays, such a strategy would be impossible because radars, satellites and sensors have become so advanced that the moment a missile was fired we could detect it.
In addition, the world now is so much more interconnected than it ever once was, meaning that the Cold War scenario of Russia vs. the USA is no longer tangible. Truly a nuclear war, were one ever to happen, would destroy the whole earth, not leaving swathes of the USA and other parts of the world left unharmed.
Alas Babylon was written in 1959 and reflects a different technological world. Specifically as regards Mark's message, it comes in the form of a telegram, an old technology that is seldom used anymore and would be extremely unlikely to be the method of choice to communicate an urgent message today because it is relatively slow. A telegram was a written message sent via telegraph wires to a destination where it was then printed and delivered to a location. If the novel were written today, people would communicate through cell phone or internet. Using a telegram would be the sign of a severe emergency, an indication that the internet and phone lines were all down, and would doubtless be commented on by the characters in the book. In this novel, it is treated as a normal occurrence.