Feelings which were believed to be virtuous, and which certainly wore the imposing garb of patriotism, impelled them with almost irresistible force against that wise neutrality which the executive government had laboured to preserve, and had persisted in preserving with wonderful and unexampled firmness.

France might, not unreasonably, indulge the hope that our government would be forced out of its neutral course, and be compelled to enter into the war as her ally.

The letter to Mazzei could scarcely fail to encourage this hope.
The suggestion had been repeatedly made, and France not only countenanced but acted on it, that the American people were ready to take part with her, and were with difficulty restrained by their government.

That the government had fallen into the hands of an English party who were the more closely attached to their favourite nation, because they were unfriendly to republicanism, and sought to assimilate the government of the United States to that of England.
Partiality to England was ingratitude to France.

Monarchical propensities were of course anti-republican, and led to a system of policy separating the United States from republican France, and connecting them with her monarchical enemies.
These sentiments were expressed in the interpolated sentence; and are intimated in terms perhaps more offensive, certainly not to be mistaken, in the letter as avowed.
Review its language.
"In place of that noble love of liberty and republican government which carried us triumphantly through the War, an Anglican, monarchical, and aristocratical party has sprung up, whose avowed object is to draw over us the substance as it has already done the forms of the British government." Could this party have been friendly--must it not have been hostile to France?
It was not only monarchical and aristocratical,--it was Anglican also.