Siska became a Airline Pilot in 1936, and joined the Czech Military Flying School shortly after, serving as a pilot until the Nazi's occupied Czechoslovakia.

He decided to leave Czechoslovakia, and when an attempt to steal an aircraft failed in January 1940, he fled through Czechoslovakia to Hungary where he was arrested by the pro-Nazi regime. Siska escaped from several prisons before making way to Yugoslavia, and then to Beirut where he enrolled in the French Foreign Legion, then the Czech Airforce in Agde, southern France. He made it to England where he joined the Free Czech Air Force.

He was shot down three times. The third occasion was on return from an op to Wilhelmshaven, on Germany's northern coast. The port engine caught fire - it was extinguished but the engine fell from the plane, making it uncontrollable, and he crashed into the North Sea.

Five of the six crewmembers escaped the plane as it sunk, and two died before they washed up on the Dutch coast six days later, and were taken prisoner. Siska received medical treatment, and he was moved to a German prison six months later. After several POW camps, he was moved to Prague to face charges of high treason and espionage against the Thrid Reich. Siska was imprisoned in Colditz for 10 months, and was moved to another camp just a day before the prison was liberated by American troops.

Siska and other relatively fit prisoners took over the camp when the German soldiers weren't present until the Americans arrived. A few weeks later, he was moved to Brussels, then England. He stayed at QVH for two years, receiving treatment for severe frostbite, before returning to Czechoslovakia.