Create inverse gradient opacity

[Nikos5C56]

Dear members, I just start learning Adobe Illustrator. I want to create this blue-ish effect on the map you see below. The layers I am using is Slovenia (layer_1) and the countries surrounding Slovenia as a sepperate layer (layer_2). I don't know if opacity is the correct method to use, so I apologize for the subject's title. Also, I don't know if layer is the right terminology for the two files I am using (Slovenia and the other countries). Any ideas on how can I achieve this effect? Many thanks.

[Ton Frederiks]

You could create a new layer between the two layers.

Create a rectangle on that layer and fill it with a gradient.

Adjust the transparency options of the gradient object (Here multiply blending mode is used).

[Nikos5C56]

That's great! Thanks very much, I will try it right now.