Although the phrase "All for one, one for all" was made famous by The Three Musketeers, it is much older than the novel, dating back at least to the early seventeenth century. It is also the unofficial motto of Switzerland.
The phrase means that the musketeers approach their profession in the spirit of brotherhood. If one of them is in danger, all the others will rush to his assistance; if the regiment is engaged on any matter, not one of them will fail to take part. The motto is proposed by d'Artagnan soon after he has joined the inseparable three musketeers of the title, Athos, Porthos and Aramis, who, despite their very different characters, are bosom friends as well as companions in arms.
The motto of the musketeers is the key to their strength in a court that is driven by faction and intrigue. The queen has many secrets from the king. The cardinal is always plotting against both of them. Even the cardinal's leading agent, Milady de Winter, has her own agenda. Against this background, the musketeers' cohesive approach allows them to succeed in their missions and to trust one another even when, as is often the case, they do not quite understand each other's methods.