This paper considers the problem of how to allocate power among competing users sharing a frequency-selective interference channel. We model the interaction between these selfish users as a non-cooperative game. As opposed to the existing iterative water-filling algorithm, which studies the myopic behavior of users, this paper studies how a foresighted user, who knows the channel state information and response strategies of its competing users, should behave. To characterize this multi-user interaction, the Stackelberg equilibrium is introduced, and the existence of this equilibrium for the investigated non-cooperative game is shown. We analyze such interactions in more detail using a simple two-user example, where we define the strategic behavior of a foresighted user as a bi-level programming problem, and derive the necessary optimality conditions. It is analytically shown that a foresighted user can improve its performance, if it has the required information about its competitors. ...