Are there any seforim that discuss why (zionistic) nationalism is not a Torah-idea?
It will be difficult to find early texts that discuss Zionism as we know it today, since the idea did not arise in its current form until the mid-19th century, but we do have some published material from Rabbi Elchonon Wasserman, who lived in the late 19th, early 20th century: (There is much more from others, including the Chazon Ish and the Brisker Rav, but I did not have time to research those sources now, and much of that material is oral.)
Here is a snippet from an article released by HaRav Wasserman ZT”L in the early 20th century entitled "The Heels of Moshiach". (Unfortunately I don't have a printed copy here, so I don't know the exact date of publication.)
הרעיון הלאומי - The Idea of Nationalism
עקבתא דמשיחא
הגה"ק-רבי אלחנן וסרמן הי"ד
הרעיון הלאומי \47
הרעיון הלאומי כמות שהוא אינו מוכר על ידי התורה ואינו אלא אליל חדש שגם
הוא עבודה זרה. במרוצת הזמן הוליד הרעיון הלאומי בן ונקרא שמו
דתי-לאומי. שם זה מורה כי התואר דתי לבדו אינו מספיק,יש להשלימו בתואר
לאומי. השם הזה לכשעצמו, מהוה כפירה באחד מיסודות האמונה. נאמר: "תורת ה'
תמימה", היינה אין היא חסרה דבר הוזהרנו "לא תוסיפו",וכל המוסיף
גורע". אם הרעיון הלאומי הוא בבחינת עבודה זרה, אזי הרעיון
הדתי-לאומי בבחינת עבודה זרה בשיתוף
The Idea of Nationalism:
The actual concept of nationalism is not recognized by the Torah, and
is merely a new idol - also amounting to Avoda Zara.(Idol Worship). Over the course
of time, the concept of nationalism "gave birth to a son" [spawned an idea; a
term] called "Religious-Nationalist". This term indicates that to be
only "Religious" is insufficient, and must be completed by appending
to it the term "Nationalist".
This term [Religious-Nationalist] stands
in contradiction to one of the foundations of our faith: "The Torah of
G-d is perfect". That is to say, the Torah itself lacks nothing. We
have been admonished "do not add" [to the Torah]; "all who add [to the
Torah] degrade it".
If the idea of nationalism itself is Avoda Zara, then the term
"Religious-Nationalist" is a form of Avoda Zara by association.
As mentioned by @Lanied Sofey in the comments above, an excellent contemporary source which delves in depth into the question of Zionism at large is The Empty Wagon: Zionism's journey from identity crisis to identity theft - well researched and sourced.
