Enable/disable RequiredValidator on client-side / CustomValidator not firing

I've got a drop-down where the user selects a Country. It is a required "field".

Next to it, there is a textfield named State. If the user selects US, then the field State is required. If the user selects e.g. Sweden, the State is not required, since Sweden has no states.

Example code:

<asp:DropDownList runat="server" ID="Country"></asp:DropDownList>
<asp:RequiredFieldValidator ControlToValidate="Country"
                runat="server" Display="Static" ErrorMessage="Required field" />

<asp:TextBox runat="server" ID="State"></asp:TextBox>
<asp:CustomValidator ClientValidationFunction="DoesntGetFiredIfStateIsEmpty"
                runat="server" Display="Static" ErrorMessage="Required field" />

<!-- SO, RATHER THIS TOGETHER WITH CONDITIONAL FIRING -->
<asp:RequiredFieldValidator ControlToValidate="State"
                runat="server" Display="Static" ErrorMessage="Required field" />

My question to you is: How can I make this CustomValidator fire validation when it is empty?

Or put simplier: How can I make a RequiredValidator fire conditionally?

Or simplest: How can I enable/disable a RequiredValidator on client-side?