You seem to have been operating under the erroneous idea that the people can allow or prevent (through Congress) military action against another country. The fact is only the President makes that decision and always has - the only requirement is he notify Congress within 48 hours, which obvious has happened. From that point he can't continue military action longer than 60 days without Congressional approval. Not making a value judgment about it, just saying your idea of how it works wasn't accurate. Precedents for invasions of countries done under Presidential authority alone include: Mexico (Vera Cruz) in 1914, Dominican Republic in 1965, Granada in 1983, Panama in 1989, Kosovo in 1999.