I screwed this one up, he told himself, as he motioned for his men to fall back.  There was no time to recall any of his squads, not in time to make a difference.  He’d misread the situation and was about to suffer for it.  The post-exercise discussions would be hellish.  I ...

His radio buzzed.  “ENDEX,” a voice said.  “I say again, ENDEX.”

Charles blinked in surprise.  The exercise had barely begun!  Why was it being terminated?

“Understood,” he said, keying his throat mike.  “ENDEX acknowledged.”

He stood up and looked around.  Salisbury Plain training area had been carefully designed to allow the various ground forces to practice their trade.  The Royal Marines, who were regularly deployed to various colony worlds, made good use of the facilities ... but now, the exercise had simply been terminated.  He saw his commandos leaving cover and walking back towards the garrison, chatting to friends among the ‘enemy’ force.  One way or another, he knew, he would never have a chance to recover from his mistake.  It would be a permanent black mark on his record.

They reached the garrison, where a handful of sergeants were hastily sorting out the various units and pointing them towards their barracks.  Charles spoke briefly to his Regimental Sergeant Major, then hastened down the corridor towards the briefing room.  It looked as though every exercise had been terminated, all commanding officers gathering to be briefed together.  Something was definitely wrong.

He took a seat in the briefing room and chatted briefly to the other officers, but it rapidly became clear that none of them knew what was going on.  The CO entered the room, waved them back into their chairs before they could salute properly, then took his place at the front of the chamber.  His face was pale, almost ashen.  Had there been a natural disaster, Charles asked himself, or had terrorists struck again?  He forced himself to relax.  No doubt they would be given the answers soon enough.

“We have made first contact – and they’re hostile,” the CO said, once the doors had been sealed.  “Several planets have been attacked by alien forces.”

He ran through a brief outline of everything they knew, which wasn't much.  Civilians had no idea how long it could take to get a message from the edge of human space to Earth, which meant that the aliens might already have invaded several more worlds.  It was quite likely that several worlds along the frontier had no idea of what was going on – and the first warning they’d get would come when alien starships materialised in their skies.