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This is answered in the Wikipedia article lede: The Tripartite Pact was directed primarily at the United States. Since the US finally entered WWII in late 1941, the pact can be considered very early, foreseeing eventual conflict with the US.
Before this, Germany already had alliances with Japan and Italy (Anti-Comintern Pact) as early as 1936. Prior to the invasion of Poland, Germany and Italy allied in the Pact of Steel. Japan was invited as a signatory, but because this pact was aimed at Britain and France, and not USSR as Japan wanted, they declined. Later when war with the US was becoming inevitable, despite their popular support for isolationism, the three parties signed the Tripartite Pact.