**Core Concept**
The "amalgam war" refers to a controversy surrounding the use of amalgam restorations in dentistry, specifically the debate over their potential health risks and quality.

**Why the Correct Answer is Right**
The controversy surrounding amalgam restorations dates back to the mid-19th century when dentists began using silver-tin amalgams for fillings. The primary concern was the potential release of mercury vapor from these restorations, which some believed could be toxic to patients. This led to a heated debate, often referred to as the "amalgam war," between dentists who advocated for the use of amalgams and those who opposed them due to concerns over mercury toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the controversy surrounding amalgam restorations is well-documented, but the exact timeline is unclear.
**Option B:** This option is incorrect because while the controversy surrounding amalgam restorations was widespread, it was not limited to the early 20th century.
**Option C:** This option is incorrect because while the controversy surrounding amalgam restorations was indeed a significant issue, it was not the primary cause of the controversy.

**Clinical Pearl / High-Yield Fact**
The "amalgam war" highlights the importance of ongoing evaluation and debate in the field of dentistry, as well as the need for careful consideration of potential health risks associated with new treatments.

**Correct Answer: D. 1850s**