Explanation:

- "is held":

- This option uses the present tense, suggesting that the belief is currently accepted.

- It implies an ongoing acceptance that fits the context of "even when war is actually in progress."

- This is the most appropriate choice for the blank space.

- "has been withheld":

- This option uses the present perfect tense and describes something that was previously held back.

- It doesn't fit the context of a belief that remains widely accepted during wartime.

- "was held":

- This option uses the past tense, indicating that the belief was accepted at some point in the past.

- It doesn't match the ongoing and current nature of the belief.

- "had been withheld":

- This option uses the past perfect tense, indicating that something was held back before a certain time in the past.

- This again doesn't match the context or meaning required for an ongoing belief.