No Cyber Maginot Line

Ideally, we'd like to stop all attacks before they succeed.  In the Real World, that's not plausible.  The game is to detect and respond.  Like at the White House:

Hackers reportedly attempted a brazen attack on a White House military network in charge of the president’s nuclear football.

US officials familiar with the incident said unidentified hackers launched an attack early last month on the network used by the White House Military Office (WHMO), an military office in charge of sensitive communications, including systems to send and authenticate nuclear strike commands. The office is also responsible for arranging presidential communications and travel. However it seems only less significant systems were targeted by an assault that was, in any case, ultimately unsuccessful.

...

Follow-up reports suggest that a dodgy email with a malicious attachment made it past perimeter defences and onto someone's desktop, where it might have been opened, and a machine infected. But this machine was quickly identified and isolated before any damage was done.

Quite frankly, that's exactly how it's supposed to work.  There's a lot of this going on, and Condition White is no longer a luxury most people can afford while they're online.