It is true that at the end of 1940, Naftali Lubinchik, a young man from Lehi went to Beirut and met there German diplomat whose name was Werner Otto von Hentig. He offered him intelligence and military support from Lehi, and in return Germany will get rid of the British rule in Palestine and make Palestine a Jewish state. It is a very dark episode but at the time Lehi decided to fight Britain at all cost. They actually saw themselves as responsible to the Jews in Palestine and to the concept of Jewish sovereignty over Palestine, and not to the Jews where ever they are. Another thing to put in mind is that the news about the systematic killing of Jews in Europe were still unclear in Palestine at the time. So they assumed it is possible to cooperate somehow with the Nazis in order to save Jews by bringing the Jews to Palestine. The German diplomat declined the offer of course. Lehi wasn't a legitimate power in the eyes of the Jewish leadership at the time and the small group was referred simply as 'terrorists'. All sources I've found are in Hebrew.

In Mandatory Palestine, there were several Jewish paramilitary groups. They varied mostly by how rabidly anti-British they were. The Haganah, the mainstream one, was largely resisting the British non-violently with illegal immigration campaigns and the like. They actually worked with the British at times against the more radical groups.

Lehi (which is a Hebrew acronym for Lohamei Herut, "Freedom Fighters") was one of the most radical ones. They formed during the beginning of WW2 in objection to Etzel (a more-radical-than-Haganah group) suspending their campaign against the British for the duration of the war, which they ended up resuming in 1944. Lehi's sole purpose, essentially, was all-out war against the British. At the time they thought that allying with Germany would have two advantages. First, they shared a common enemy in the British, and fighting them was Lehi's reason for existing. Second, Germany might be amenable to the idea of having somewhere convenient to exile Jews to. Obviously, nothing like that was ever worked out, and Lehi just carried on as a very radical anti-British Jewish underground until Israeli Independence.

the post op is talking about http://www.reddit.com/r/vexillology/comments/2y0jt5/flag_of_lehi_a_jewish_terrorist_group_that/