Most international diplomacy is down on the bilateral level examples of this include treaties between two countries, exchanges of ambassadors, and state visits, among other things. The alternatives to bilateral relations are multilateral relations, which involve many states. Unilateralism is when one state acts on its own.
There has long been a debate on the merits of bilateralism versus multilateralism. The first rejection of bilateralism as dangerous came after the First World War when many thought that the complex series of bilateral treaties had made war all but inevitable. This lead to the creation of the multilateral League of Nations.
A similar reaction against bilateral trade agreements occurred after the Great Depression, where it was argued that they helped produce a cycle of rising tariffs that deepened the downturn. Thus after the Second World War the west turned to multilateral agreements such as GATT.
Despite the high profile of modern multilateral systems such as the United Nations and the World Trade Organization most effective diplomacy is still done at the bilateral level. Bilateralism has a flexibility and ease the compromise dependent multilateral systems lack.