"Before" here means "until." It's like saying:
- When will Korea be able to nuke a US city?
- How long will it take for Korea to be able to nuke a US city?
- How much time will go by before Korea can nuke a US city?
1. It's very informal English. There's no verb, but it is natural.
2. There are two Koreas: North Korea and South Korea. To make sense, you need to specify which.
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