This is obviously true, but it's not as simple as it may seem.

The body has a complex energy regulation system and hunger, energy consumption via metabolism and energy storage are interlaced. It's not as simple as "energy in" = independent variable.

It has a preference to store (instead of immediately use) calories from carbohydrates (due to insulin). So a diet low in total calories but still high in carbs is going to be painful, and very frequently fails.

Read Gary Taubes' book, people.