The United States and British troops should not march in the parade because of the inhumane acts that Stalin committed during his time of reign. The celebration commemorates the fall of Nazi Germany and the horrendous acts of Hitler and the Nazi Party. It is counterintuitive to criticize and celebrate the downfall of one inhumane group and at the same time praise a man who committed crimes that were just as bad and sometimes worse. The troops should not march if Stalin is celebrated because they would be condoning the acts of lunatic. In a time when the UN is trying to limit inhumane acts in the world two of its most important members should not celebrate a leader that is known for his acts against other men.
The only downfall of not marching in the celebration is that it will hurt U.S. relations with Russia. The relations of the two countries are declining quickly. This due to the U.S. blowing up Russian gas lines during the Clinton administration and Russia supporting Iran's bids for nuclear technology. This may lead to a potential war between the country. It could be a physical war or it could be an arms race such as the Cold War.
In conclusion the United States and Britain should not march because it is better to hurt relations with Russia than to condone inhumane acts against man in the rest of the world.