In August, when President Donald Trump's lawyers persuaded him to refrain from attacking independent counsel Robert Mueller publicly — he had many times called Mueller's investigation a "witch hunt" — they also told him that the investigation was not aimed at him and not to worry because it would be over by Thanksgiving.

These are the same lawyers who revealed a fissure in the strategy of the president's legal team. On one side are presidential lawyers who want to cooperate fully with Mueller because they are convinced that the president has nothing to fear from the public revelation of truthful information.

On the other side are presidential lawyers who believe that the office of the presidency is being diminished by its subservient surrendering of all demanded documents and emails to Mueller's investigators — materials that skilled prosecutors might somehow be able to use against the president or members of his inner circle someday.

Adding to this is the presence of Gen. John Kelly, the White House chief of staff, a non-lawyer, who serves as the referee when the president's lawyers disagree. That disagreement was heightened on Thanksgiving eve when a letter arrived at the White House. It must have been received as if it were a thunderbolt and no doubt generated bitter reminders of the "over by Thanksgiving" remark made to Trump personally.

Here is the back story.

When the government is investigating or prosecuting a group of people who have some common bond or prior relationship or mutual interest, it is not uncommon for their lawyers to enter into a joint defense agreement. This type of agreement is a written contract that generally governs the relationship of the potential defendants to one another and their joint relationship to the government. Though no two agreements are identical, they generally provide for the sharing of communications to and from the government, the sharing of evidence and a joint defense.

So if one of the lawyers in the agreement receives a request from the government for documents from that lawyer's client, that lawyer will share the request and copies of the surrendered documents with the other lawyers who have signed the agreement.

The joint defense established by the agreement — we will sink or swim together — prohibits any person whose lawyer has signed the agreement from pointing a finger at any other person whose lawyer has signed the agreement. This united front often strengthens the defense team and frustrates the government. These agreements are common, lawful and binding until any potential defendant withdraws.

Lawyers must be very careful about what they have agreed to do, because a lawyer's first and core loyalty is to her or his client, not to the group of folks represented by fellow counselors who have signed the agreement.

In the Mueller investigation, lawyers for some of the major targets — but not all — signed such an agreement. Those who did include lawyers for the president and lawyers for his former national security adviser, Michael Flynn.