Should a citizen be able to sue the armed forces, For any cause?

In short, no. The armed forces are part of the US Government which has sovereign immunity. The US Government cannot be sued unless a law is in place that waives sovereign immunity. There are varying degrees of sovereign immunity applicable to state and local government as well. Although not found in the Constitution, this concept is based on British Common Law.

So technically a citizen can sue the government, but they require permission from the government first. Japanese-Americans interned during WWII are one of the very few successful examples of this.

For any cause? No. For certain causes? Yes.
The question is SHOULD, not MAY. CAPT Kevin B. and SFC Marc W. have answered MAY.

I believe, in certain cases, citizens suing the government is appropriate. These cases would require the individual to show that they suffered some form of true harm (beyond simply being stressed out or "mental anguish" over possible - yet still unrealized - effects of a policy) from a governmental action. Further, they would have to show that the government either A) failed to do uts due diligence in checking who would be harmed, or B) did its due diligence, recognized that a person or group of persons would be harmed, and proceeded anyaway.

Example 1: Army Corps of Engineers builds a levee on the upper Mississippi. Their analysis shows that this levee will save 25,000 homes in a major urban center, but destroy 500 homes in a rural area downriver. Yes, those 500 should be able to sue. The action was taken KNOWING IT WOULD HARM those 500. It was still the right action, but that does notabsolve the government from responsibility, IMO.

Example 2: FDA approves a vaccine which has undergone rigorous and extensive testing. It has proven safe and effective. 10 years later, after full implementation, backwards looking research shows that it causes birth degects in 0.5% of the newborn population, based on mothers who recieved the vaccine while pregnant and also having a rare specific gene sequence. FDA immediately bans the vaccine for all pregnant women until testing can be done to deyermine prsence of said gene sequence. In this case, no. FDA did its due diligence, but something slipped through the cracks. This rare anomaly isn't something they should reasonably be expected to find and predict. Further, once it was known, they immediately took action to prevent further harm.

That is the way I think it SHOULD be.