Another reason this is a historic week — and what President Trump should really worry about — is that lots of different Republicans have been turning on him over different topics.

Why it matters: A former Trump aide who asked to be described as "a Trump ally" told Axios that the sudden wave of criticism from the Hill over Syria and Mattis should scare the president because he would desperately need these lawmakers' support during a possible impeachment battle.

"Once Republican lawmakers start rebuking the president publicly like this over policy, it makes it easier for them to say: 'It's not just Mueller or ethics. There are other concerns.' Then it's a slippery slope."

Catch up quick: In an unusually harsh statement, Senate Majority Leader Mitch McConnell said he was "distressed" about Mattis' departure: "It is regrettable that the president must now choose a new Secretary of Defense. But I urge him to select a leader who shares Secretary Mattis's ... principles."

Sen. Ben Sasse (R-Neb.) said after the Mattis announcement: "This is a sad day for America."

Bloomberg columnist Eli Lake: "Donald Trump may not know it yet, but his presidency is collapsing."

"As long as [Mattis] served the president, reluctant Republicans could point to the Pentagon and say: If Mattis supports Trump, then so do I. They can no longer do that."

Be smart: We talked all day yesterday with Republican officials, operatives and advisers who are truly scared for America.

But it's telling that few have the courage to say it publicly.

What to watch: There’s a working assumption inside the White House that Trump will be impeached by the House. He would then need a rock-solid base of 34 Republican senators to refuse to remove him from office (which takes 67 votes).

So nothing matters more to Trump than keeping his base happy and loyal.

Go deeper: As a sign of the mood inside, officials at both ends of Pennsylvania Avenue tell us that Trump is complaining about his incoming chief of staff, Mick Mulvaney, in conversations inside the West Wing and with Capitol Hill.