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The implication is that "even if they offer peace only with a view to deceiving thee, this [offer of] peace must be accepted, since all judgment [of their intentions] must be based on outward evidence alone" (Razi): in other words, mere suspicion cannot be made an excuse for rejecting an offer of peace.
Lit., "and by the believers": thus signifying the visible means (wasitah) by which God succoured the Prophet.
In working for peace there may be a certain risk of treachery on the other side. We must take that risk: because the men of Faith have Allah's aid to count upon and the strength of the united body of the righteous.