"There are no permanent allies, no permanent friends, only permanent interests", is a clichÃ© which quite often describes the behavior pattern of nations. This is because it is the interest of a nation that governs its behavior in a given situation. Therefore the interests of a nation remain paramount. The status allies and enemies keep changing depending on what suits the nations interest best. Modern history has several examples to prove the above stated point.
The Afghanistan war of 1979 is a good example to prove it. Soviet Union had many interests in Afghanistan and one of them was to spread the theory of communism and make it one of its satellite nations. The United States was pursuing a policy of containment and it indirectly aided the rebel forces of Afghanistan, i.e. the Taliban and Mujahhidins, to overthrow the communist regime. United States allied with Afghanistan along with Iran, China and Pakistan to carry out its objectives. Although the war ended in a stalemate, but the participating nations were involved because it served their interests. After 9/11 United States developed an interest of combat against terrorism and it suspected its former ally, Afghanistan or the Taliban, to be involved in the attacks on the WTO twin towers. The super power attacked Afghanistan due to a change in its interests and in fact it received support from its former enemy, Russia. In course of a decade, at first United States supported a nation and in the end attacked and destroyed it. It took part in both occasions purely due to the reason of interests. It allied with nations that had similar interests and opposed those who had interests that were contradicting. Another example from modern History is the behavior of United States during the Iraq-Iran war of the 1980's.
The Iraq â€“ Iran war a typical example to show that interests are the main reason that a nation gets involved in any given situation.