The phrase "diplomatic line" is correct and can be used in written English.
It refers to a particular stance or approach taken by a government or organization in their diplomatic relations with other countries or entities. Example: The government's diplomatic line on the current trade dispute is to maintain open channels of communication while standing firm on protecting national interests.
In the Netherlands, officials also sought to tread a delicate diplomatic line, condemning the attack on their diplomat while trying not to widen the rift with the Russian government by accusing it of complicity.
But it is a fine diplomatic line to walk.
Many Turkish Islamists despise Israel, but Erdogan and the AK adopted a more diplomatic line.
On Khan's proposed fares freeze, Brown takes a diplomatic line: "It's early days.
The Mujahedeen fears the administration wants to keep a diplomatic line open to Iran in the escalating nuclear dispute.
On recognising and spelling countries, the Factbook follows the US diplomatic line, and the recommendations of the United States Board on Geographic Names.
But Hanoi is also carefully hedging its bets by keeping diplomatic lines open to Beijing.
That summer, following the German invasion of the Soviet Union, Roosevelt again ignored established diplomatic lines of authority and dispatched Harry Hopkins to Moscow to meet with Joseph Stalin to discuss future American assistance.
Although Saudi Arabia remains the GCC state that most aggressively counters Iran, it appears that Riyadh is making efforts to keep diplomatic lines with Iran open, while continuing to wage proxy wars aimed at countering Tehran's influence in the Arab world, most notably in Syria.
An old-fashioned, tanks and troops invasion would require the West to respond much more vigorously, and at much greater cost and risk to itself, than it has done so far, not least because its leaders have promised as much.This is a diplomatic "red line" that (like most red lines) no one wants to admit Russia has crossed, unless boots and uniforms on the ground oblige them to.
That complicates matters: you need to be ready for a full-fledged campaign on the operational level and have the diplomatic backing lined up as well.
