<a href="https://en.wikipedia.org/wiki/World_War_I" id="mwdA" rel="mw:WikiLink" title="World War I">World War I</a>: <a href="https://en.wikipedia.org/wiki/U-boat_campaign" id="mwdQ" rel="mw:WikiLink" title="U-boat campaign">U-boat campaign</a>: Two days after the United Kingdom had declared war on Germany over the German invasion of Belgium, ten German <a href="https://en.wikipedia.org/wiki/U-boat" id="mwdg" rel="mw:WikiLink" title="U-boat">U-boats</a> leave their base in <a href="https://en.wikipedia.org/wiki/Heligoland" id="mwdw" rel="mw:WikiLink" title="Heligoland">Heligoland</a> to attack <a href="https://en.wikipedia.org/wiki/Royal_Navy" id="mweA" rel="mw:WikiLink" title="Royal Navy">Royal Navy</a> warships in the <a href="https://en.wikipedia.org/wiki/North_Sea" id="mweQ" rel="mw:WikiLink" title="North Sea">North Sea</a>.