Reilly would have an even greater success in January 1904, when he and Chinese engineer acquaintance Ho Liang Shung allegedly stole the Port Arthur harbour defence plans for the Japanese Navy. Guided by these stolen plans, the Japanese Navy navigated by night through the Russian minefield protecting the harbour and launched a surprise attack on Port Arthur on the night of 8–9 February 1904 (Monday 8 February – Tuesday 9 February). However, the stolen plans did not help the Japanese much. Despite ideal conditions for a surprise attack, their combat results were relatively poor. Although more than 31,000 Russians ultimately perished defending Port Arthur, Japanese losses were much higher, and these losses nearly undermined their war effort.