Forewarned is forearmed
Definition 1 of 1
Pro Tip 1/3Use in Caution
The phrase is often used to advise caution and prepare for potential problems. 
When hiking in snake territory, forewarned is forearmed—bring a snake bite kit.
Pro Tip 2/3Preparation Context
It implies taking actions or precautions after being informed of possible future events. 
With the storm forecasted, we stored water and food, as forewarned is forearmed.
Pro Tip 3/3Metaphoric Use
Used metaphorically, it doesn't always refer to literal arming but to mental readiness. 
Before the debate, I researched all topics; forewarned is forearmed.