Keterlibatan Amerika Serikat dalam Perang Dunia dimulai pada bulan April 1917, dua setengah tahun setelah Presiden Woodrow Wilson menyatakan bahwa Amerika Serikat akan tetap netral sepanjang Perang Dunia I. Rakyat Amerika Serikat tidak tahu bahwa perang akan pecah di Eropa pada musim panas 1914, dan puluhan ribu wisatawan dikejutkan oleh peristiwa ini. Pemerintah A.S. di bawah kepemimpinan Wilson mengumumkan bahwa "pikiran maupun niat" Amerika Serikat tidak berpihak.[1] Selain unsur-unsur Anglofilia yang mendukung Britania, opini publik Amerika Serikat awalnya netral. Posisi netral sangat kuat di kalangan warga Irlandia Amerika, Jerman Amerika, dan Swedia Amerika,[2] serta para pemimpin gereja dan wanita. Di sisi lain, sebelum perang pecah, opini publik A.S. terhadap Jerman jauh lebih negatif dibandingkan opini mereka terhadap negara lain di Eropa.[3] Rakyat memandang Kekaisaran Jerman sebagai musuh setelah mengetahui aksi kekerasan di Belgia tahun 1914, dan penenggelaman kapal penumpang RMS Lusitania tahun 1915 yang berlawanan dengan hukum internasional. Wilson membuat semua keputusan penting dan menjalankan ekonomi secara damai, namun tetap memberi pinjaman besar kepada Britania dan Prancis. Agar Amerika Serikat tidak dianggap memiliki ancaman militer, Wilson membuat persiapan perang yang sedikit dan membentuk militer berukuran kecil meski ditekan untuk membuat persiapan besar. Namun demikian, ia justru memperbesar ukuran Angkatan Laut Amerika Serikat.
Pada awal 1917, Jerman memutuskan untuk melanjutkan perang kapal selam total terhadap semua kapal dagang yang menuju ke Britania Raya. Keputusan ini hampir pasti berarti perang melawan Amerika Serikat. Jerman juga menawarkan persekutuan militer dengan Meksiko dalam telegram Zimmermann. Bocornya telegram tersebut dan penenggelaman kapal-kapal Amerika Serikat di Atlantik Utara oleh U-boat Jerman membuat rakyat A.S. meradang. Wilson meminta "perang untuk mengakhiri perang" yang akan "membuat demokrasi berdiri di dunia tanpa gangguan" kepada Kongres. Kongres memutuskan untuk menyatakan perang melawan Jerman pada tanggal 6 April 1917.[4] Tanggal 7 Desember 1917, Amerika Serikat menyatakan perang melawan Kekaisaran Austria-Hungaria.[5][6]
^Attributed afterwards. See: "Ten Years After". Lawrence Journal World. July 28, 1924. Diakses tanggal July 24, 2012.
Original quote, "be neutral in fact as well as in name ... be impartial in thought as well as in action". See: "The American People and the Great War". The Independent. August 31, 1914. Diakses tanggal July 24, 2012. and Message to Senate
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Historiografi
Doenecke, Justus D. "Neutrality Policy and the Decision for War." in Ross Kennedy ed., A Companion to Woodrow Wilson (2013) pp: 243-69 Online; covers the historiography