Has an army ever not cancel its preplanned attack after a security breach?

Question

In an episode of the Star Wars show Andor a military leader (through a high ranking spy) becomes aware that his planned attack is compromised, the enemy knows about it and has plans to deal with that attack. However, this leader does not cancel the attack, in order to keep his information source secret. Has something similar ever happened in history? I'm interested in deliberate actions to withhold vital information (with the main purpose to guard the source of the information) that lead to loss of men, not accidental cases (like Stalin not believing Hitler would attack on 22 June 1941).

I read that the Allies were extra careful to guard the fact that they broke the Ultra machine in World War II, but haven't actually lead their troops into losing battle to keep their secret. There are conspiracy theories about prior Allied knowledge on the Pearl Harbour attacks, but even in those cases the reason to let enemy succeed was not to guard intelligence sources.

Answer 1

Generally speaking, it is extremely bad policy to continue an attack after the plan had been breached. Even the mere risk of a security breach is good reason to cancel an attack forthwith.

It is possible, and has happened:

An example of this is the Mechelen incident. An officer with his pilot flew into Belgium in bad weather, on 10 January 1940. He carried, highly illegal, the (original) operation plans for Fall Gelb (Case Yellow) with him. The Belgians took them prisoner, and were able to secure at least part of the plan. Germany didn't know that, so they assumed the whole plan was now known to the enemy. They changed the plan accordingly, adopting the von Manstein plan, which became the new Case Yellow plan.

The attack went through, as soon as was possible (Fall Gelb had been postponed at least 30 times due to various causes). The Allies more or less responded to the old plan, but the Germans came from a very different direction (Ardennes) this time.

In the second example the Germans were aware the Enigma code might have been breached, but they weren't certain. So they kept sending out their most valuable submarines, considering the risk worth it.

The Milch Cow, Type XIV, was a dedicated supply submarine. It was designed (not converted) to carry supplies to other submarines while they were at sea on station. That greatly increased their operational range. Those supply subs were the number one priority over all other U-Boats for the Allies.

At that time the Enigma code had been broken. The Allies were able to locate each and every one of the German supply subs. Sinking them immediately, at every first opportunity, would have been possible. However, doing that wasn't prudent. The Germans would immediately understand the Enigma code was broken.

The Allies decided to let some supply submarines live, and carry out their duties. They would resupply U-Boats, which would then sink more Allied ships than they otherwise would be able to.

Eventually, the Milch Cows were all hunted down and sunk, but not as soon as possible. That was at the cost of Allied lives, but in the long run it did save more lives.