<a class="mw-redirect" href="https://en.wikipedia.org/wiki/U-boat_Campaign" id="mwnQ" rel="mw:WikiLink" title="U-boat Campaign">U-boat Campaign</a>: The Imperial German Navy institutes unrestricted submarine warfare in the waters around Great Britain and Ireland.<sup about="#mwt31" class="mw-ref reference" data-mw='{"name":"ref","attrs":{},"body":{"id":"mw-reference-text-cite_note-4"}}' id="cite_ref-4" rel="dc:references" typeof="mw:Extension/ref"><a href="./February_18#cite_note-4" id="mwng" style="counter-reset: mw-Ref 4;"><span class="mw-reflink-text" id="mwnw">[4]</span></a></sup>
<span class="mw-reference-text" id="mw-reference-text-cite_note-4"><cite about="#mwt28" class="citation book cs1" id="CITEREFPotterRoger_FredlandHenry_Hitch_Adams1981">Potter, Elmer Belmont; Roger Fredland; Henry Hitch Adams (1981). <i id="mwBWI">Sea Power: A Naval History</i>. Naval Institute Press. p.<span id="mwBWM" typeof="mw:Entity"> </span>223. <a class="mw-redirect" href="./ISBN_(identifier)" id="mwBWQ" rel="mw:WikiLink" title="ISBN (identifier)">ISBN</a><span id="mwBWU" typeof="mw:Entity"> </span><a href="./Special:BookSources/0-87021-607-4" id="mwBWY" rel="mw:WikiLink" title="Special:BookSources/0-87021-607-4"><bdi id="mwBWc">0-87021-607-4</bdi></a>.</cite><span about="#mwt28" class="Z3988" id="mwBWg" title="ctx_ver=Z39.88-2004&amp;rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&amp;rft.genre=book&amp;rft.btitle=Sea+Power%3A+A+Naval+History&amp;rft.pages=223&amp;rft.pub=Naval+Institute+Press&amp;rft.date=1981&amp;rft.isbn=0-87021-607-4&amp;rft.aulast=Potter&amp;rft.aufirst=Elmer+Belmont&amp;rft.au=Roger+Fredland&amp;rft.au=Henry+Hitch+Adams&amp;rfr_id=info%3Asid%2Fen.wikipedia.org%3AFebruary+18"></span></span>