Essentially user5751924's answer, but expanded with textual sources.
You are not referring to suicide per se, well you could be, but the jurisprudence does not define it to be suicide at least to how I understand it.
Sourcing from Book 23, Hadith 118, Sahih al-Bukhari, Book of Funerals (Al-Janaa'iz):
حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، حَدَّثَنَا أَبُو الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم " الَّذِي يَخْنُقُ نَفْسَهُ يَخْنُقُهَا فِي النَّارِ، وَالَّذِي يَطْعُنُهَا يَطْعُنُهَا فِي النَّارِ ".
The Prophet (ﷺ) said, "He who commits suicide by throttling shall keep on throttling himself in the Hell Fire (forever) and he who commits suicide by stabbing himself shall keep on stabbing himself in the Hell-Fire."
Clearly this implies willingness to end's ones own life, intent has importance, a person does not (usually) unwillingly stab himself, etc. I would posit this is the traditional concept of suicide for most people, you have the intent for death.
When you use the phrase, "people who kill themselves to save a nation or piece of land" this does not fall into any concept promote by hadith (Sunni or Shia for that matter) nor the Qur'an. A martyr defined by textual sources, is someone who ends up dead due to the result of something noble. It is not defined as someone who seeks death in order to be labeled a martyr, by willingly killing themselves. It screams semantics here, what you describe does not mean what you think it means.
Sourcing from Book 16, Hadith 38, Jami` at-Tirmidhi, The Book on Blood Money:
" مَنْ قُتِلَ دُونَ مَالِهِ فَهُوَ شَهِيدٌ وَمَنْ قُتِلَ دُونَ دِينِهِ فَهُوَ شَهِيدٌ وَمَنْ قُتِلَ دُونَ دَمِهِ فَهُوَ شَهِيدٌ وَمَنْ قُتِلَ دُونَ أَهْلِهِ فَهُوَ شَهِيدٌ "
"Whoever is killed over his wealth then he is a martyr, and whoever is killed over his religion, then he is a martyr, and whoever is killed over his blood, then he is martyr, and whoever is killed over his family, then he is martyr."
Perhaps it is the amount of novels that glamorize death, such as the final scene where the protagonist is forced into a dichotomous system of saving the other important figure at the cost of themselves. However, in such novels the protagonist generally speaking does not seek death from the start, it just becomes the inevitable ending based on the given events. The hadith above works in the same manner, it does not call for a person to set their eyes on letting their life end by dying for their religion, rather if it happens that they end up dying, they are a martyr.
First hadith quotation source: https://sunnah.com/bukhari/23/118
Second hadith quotation source: https://sunnah.com/tirmidhi/16/38